Withdrawals before any draw

C1 1968/69 FC Levski (Sofia) (Bul), Ferencváros TC (Hun),
       Dinamo Kiev (URS), Ruch Chorzów (Pol) for politic reasons
C2 1968/69 FC Spartak (Sofia) (Bul), 1.FC Union Berlin (GDR),
       Gornik Zabrze (Pol), Dinamo Moskva (URS) for politic reasons
C2 1978/79 Sepsi-78 Seinäjoki (Fin) for financial reasons

Withdrawals before any play

C4 1956/58 Vienna XI (Aut) (FC Barcelona (Esp), KBU (København) (Den))
C4 1956/58 Stadtauswahl Köln (FRG) (Leipzig XI (GDR), Lausanne-Sports (Sui))
C1 1958/59 Olympiakos Peraías (Gre) (wo Besiktas Istanbul (Tur))
C1 1958/59 Manchester United (Eng) (wo BSC Young Boys (Sui))
C1 1959/60 KuPS (Kuopio) (Fin) (wo Eintracht Frankfurt (FRG))
C1 1960/61 CCA Bucuresti (Rom) (wo Spartak Hradec Kralove (Tch))
C1 1960/61 Glenavon FC (Nir) (wo Wismut Karl-Marx-Stadt (GDR))
C2 1962/63 Olympiakos Peraías (Gre) (wo Hibernians FC (Paola) (Mlt))
C1 1966/67 17 Nëntori (Tiranë) (Alb) (wo Vålerengens IF (Nor))
C1 1967/68 KS Dinamo (Tiranë) (Alb) (wo Braunschweiger Eintracht (FRG))
C1 1968/69 FC Carl Zeiss Jena (GDR) (wo Crvena Zvezda Beograd (Yug))
C2 1968/69 Györi Vasas ETO (Hon) (wo Dinamo Bucuresti (Rom))
C4 1968/69 KB (København) (Dan) (wo 1.FC Lokomotive Leipzig (GDR))
C4 1968/69 Union Luxembourg (Lux) (wo Újpesti Dózsa (Hun))
C3 1971/72 KS Vllaznia (Shkodër) (Alb) (wo SK Rapid Wien (Aut))
C3 1972/73 HIFK (Helsinki) (Fin) (wo Hvidovre IF (Den))
C1 1974/75 Omonia (Lefkosía) (Cyp) (wo Cork Celtic (Irl))
C2 1974/75 EN Paralímni (Cyp) (wo Avenir Beggen (Lux))
C3 1974/75 Pezoporikos (Lárnaka) (Cyp) (wo Dukla Praha (Tch))
C2 1979/80 KS Vllaznia (Shkodër) (Alb) (wo Dinamo Moskva (URS))
C1 1983/84 KS Vllaznia (Shkodër) (Alb) (wo Hamburger SV (FRG))

Withdrawals during the season

C4 1968/69 Hamburger SV (FRG) (wo Goztepe (Tur))
C1 1982/83 17 Nëntori (Tiranë) (Alb) (wo Dinamo Kiev (URS))
C1 1986/87 APOEL (Lefkosía) (Cyp) (wo Besiktas Istanbul (Tur))

Withdrawals after first leg

C1 1961/62 Linfield FC (Nir) (1st leg lost 0-3 against ASK Vorwärts (Berlin) (GDR))
C4 1961/62 Lausanne-Sports (Sui) (1st leg lost 3-4 against Valencia CF (Esp))
C4 1962/63 Basel XI (Sui) (1st leg lost 0-3 against Bayern München (FRG))
C1 1965/66 Derry City (Nir) (1st leg lost 0-9 against RSC Anderlecht (Bel))
C3 1971/72 Hallescher FC Chemie (GDR) (1st leg drawn 0-0 against PSV Eindhoven (Ned))
C2 1993/94 RAF Yelgava (Geo) (1st leg won 1-0 against HB (Tórshavn) (Far)) (2nd leg awarded 0-3)

Disqualifications before first leg

C3 2006/07 Feyenoord (Rotterdam) (Ned) (because of problems with fans during the previous round)
C3 2013/14 Fenerbahçe SK (Istanbul) (Tur) (because of bribery allegations during previous domestic league),
           replaced by APOEL FC (Nicosia) (Cyp)
C3 2021/22 Spartak Moskva (Rus) (suspension due to the Russian invasion of Ukraine)

Disqualifications after first leg

C3 1971/72 Panionios Athens (Gre) (1st leg lost 0-6 against Ferencváros TC (Hun))
C1 1987/88 KS Partizani (Tiranë) (Alb) (1st leg lost 0-4 against SL Benfica (Por))

Disqualifications after second leg

C1 1993/94 Dinamo Tbilisi (Geo) (against Linfield FC (Nir))
           for bribery allegations
C1 1995/96 Dinamo Kiev (Ukr) (against AaB (Aalborg) (Den))
           for bribery allegations (note: following match played by Dinamo Kiev in League stage then cancelled)
C3 2007/08 FK Partizan (Beograd) (Ser) (against HSK Zrinjski (Mostar) (Bih))
           because of problems with fans during the first round
C3 2011/12 Olympiakos Volou (Gre) (against FC Differdange 03 (Lux))
           because of bribery allegations during previous domestic league
C1 2013/14 FC Metalist Kharkiv (Ukr) (against PAOK FC (Thessaloniki) (Gre))
           because of bribery allegations during previous domestic league
C3 2013/14 Besiktas JK (Istanbul) (Tur) (against Tromsø IL (Nor))
           because of bribery allegations during previous domestic league

Match awarded before or without playing

C4 1960/61 Lausanne-Sports (Sui) (match awarded 0-2 against Hibernian (Sco))
C1 2020/21 KF Drita Gjilan (Kos) (match awarded 0-3 against Linfield FC (Nir))
           because two Drita members were tested positive for SARS-2 coronavirus and the whole team being put into quarantine
C1 2020/21 SK Slovan Bratislava (Svk) (match awarded 0-3 against KÍ Klaksvík (Far))
           because two Slovan members were tested positive for SARS-2 coronavirus and the whole team being put into quarantine
C3 2020/21 FC Prishtina (Kos) (match awarded 0-3 against Lincoln Red Imps FC (Gib))
           because several Prishtina members were tested positive for SARS-2 coronavirus and the whole team being put into quarantine

Leg awarded before or without playing

C3 1975/76 SS Lazio (Ita) (1st leg awarded 0-3 against FC Barcelona (Esp))
       (because of possible political demonstrations against Spain)
C1 2010/11 FC Santa Coloma (And) (1st leg awarded 0-3 against Birkirkara FC (Mal))
       (because of poor pitch conditions)
C3 2020/21 FK Qarabağ Ağdam (Azr) (2nd leg awarded 0-3 against Villarreal CF (Esp))
       (because several Qarabağ players were tested positive for SARS-2 coronavirus and the club did not have enough ones to play the game)
C4 2021/22 Tottenham Hotspur FC (Eng) (2nd leg awarded 0-3 against Stade Rennais FC (Fra))
       (because several Tottenham Hotspur FC players were tested positive for SARS-2 coronavirus and the club did not have enough ones to play the game)

Leg awarded after playing

C3 1977/78 AC Fiorentina (Ita) (1st leg drawn 0-0 but awarded 0-3
           against FC Schalke 04 (FRG)) (because of ineligible player fielded)
C2 1987/88 Hajduk Split (Yug) (2nd leg won 2-0 but awarded 0-3
           against Olympique de Marseille (Fra)) (because of supporters problems)
C3 1989/90 Ajax (Ned) (2nd leg stopped when 1-1 and awarded 0-3
           against FK Austria Wien (Aut))
C1 1990/91 AC Milan (Ita) (2nd leg stopped when 0-1 and awarded 0-3
           against Olympique de Marseille (Fra)) (because Milan refused to
           reenter on the field after projectors problems)
C1 1990/91 Dynamo Dresden (GDR) (2nd leg stopped when 1-2 and awarded 0-3
           against Crvena Zvezda Beograd (Yug))
C1 1992/93 VfB Stuttgart (Ger) (2nd leg lost 1-4 but awarded 0-3
           against Leeds United (Eng) and then play-off 1-2) (because of ineligible player fielded)
C3 1992/93 PAOK Thessaloníki (Gre) (2nd leg stopped when 0-2 and awarded 0-3
           against Paris Saint-Germain (Fra)) (because of supporters problems)
C3 1994/95 CSKA (Sofia) (Bul) (1st leg won 3-2 but awarded 0-3
           against Juventus FC (Ita)) (because of ineligible player fielded)
C2 1995/96 Lantana Tallinn (Est) (1st leg won 2-1 but awarded 0-3
           against DAG-Liepaya (Lat)) (because of ineligible player fielded)
C3 1995/96 Örebro SK (Swe) (2nd leg drawn 1-1 but awarded 0-3
           against Avenir Beggen (Lux)) (because of ineligible player fielded)
C1 1997/98 Paris-Saint-Germain (Fra) (1st leg lost 2-3 but awarded 0-3
           against Steaua Bucarest (Rom)) (because of suspended player fielded)
C2 1997/98 Dinamo Batumi (Geo) (1st leg won 4-2 but awarded 0-3
           against Ararat Yerevan (Arm)) (because of ineligible player fielded)
C1 1998/99 KS Vllaznia (Shkodër) (Alb) (1st leg lost 0-1 but awarded 0-3
           against Dinamo Tbilisi (Geo)) (because of ineligible player fielded)
C3 1998/99 AC Fiorentina (Ita) (2nd leg stopped when 2-1 and awarded 0-3
           against Grasshopper-Club (Zürich) (Sui))
           (because a referee has been injured by a firecracker)
C3 2000/01 Crvena Zvezda Beograd (Yug) (2nd leg lost 3-5 but awarded 0-3
           against Celta de Vigo (Esp)) (because of two suspended players fielded)
C1 2001/02 Maccabi Haïfa (Isr) (2nd leg won 4-0 but awarded 0-3
           against FC Haka (Valkeakoski) (Fin)) (because of ineligible player fielded)
C3 2001/02 IF Elfsborg (Borås) (Swe) (1st leg won 3-1 but awarded 0-3
           against JK Trans Narva (Est) (because of ineligible player fielded)
C3 2001/02 UFC Rapid Bucuresti (Rom) (2nd leg lost 0-1 but awarded 0-3
           against Paris Saint-Germain FC (Fra) (because of lights problems)
C1 2004/05 PAOK FC (Thessaloniki) (Gre) (1st leg lost 1-2 but awarded 0-3
           against Maccabi Tel-Aviv (Isr) (because of ineligible player fielded)
C1 2004/05 AS Roma (Ita) (1st leg stopped when 0-1 but awarded 0-3
           against FC Dynamo Kyiv (Ukr) (because a referee has been injured by a lighter)
C1 2004/05 FC Internazionale (Ita) (2nd leg stopped when 0-1 but awarded 0-3
           against AC Milan (Ita) (because of fireworks on the pich)
C3 2005/06 Zepce Limorad (Bih) (1st leg drawn 1-1 but awarded 0-3
           against Baskimi Kumanovo (Mkd) (because of ineligible player fielded)
C3 2009/10 FC Dinamo Bucuresti (Rom) (1st leg stopped when 0-2 and awarded 0-3
           against FC Slovan Liberec (Svk)) (because of supporters problems)
C3 2010/11 FC Atyrau (Kzk) (1st leg lost 0-2 but awarded 0-3
          against Györi ETO FC (Hun) (because of ineligible player fielded)
C3 2010/11 FK Baku (Azr) (1st leg won 2-1 but awarded 0-3
           against FK Buducnost Podgorica (Mot) (because of ineligible player fielded)
C3 2011/12 FC Sion (Sui) (1st leg drawn 0-0 but awarded 0-3; 2nd leg won 3-1 but awarded 0-3)
           against Celtic FC (Glasgow) (Sco) (because of ineligible player fielded)
C3 2012/13 FC Arsenal Kyiv (Ukr) (1st leg won 3-0 but awarded 0-3
           against ND Mura 05 (Svn) (because of ineligible player fielded)
C3 2012/13 Anorthosis FC Famagusta (Cyp) (2nd leg stopped when 0-3 but awarded 0-3
           against FC Dila Gori (Geo)) (because of supporters problems)
C3 2014/15 Legia Warszawa (Pol) (2nd leg won 2-0 but awarded 0-3
           against Celtic FC (Glasgow) (Sco) (because of ineligible player fielded)
C3 2015/16 FK Kukësi (Alb) (1st leg stopped when 1-2 but awarded 0-3
           against KP Legia Warszawa (Pol) (because of a Legia player was hit in the head by an object thrown from the crowd)

Games cancelled and replayed

C1 1971/72 1st leg won 7-1 by Borussia Mönchengladbach (FRG) against Internazionale (Ita) 
           cancelled because Roberto Boninsegna was hit by an empty beer can (or so he claimed) and carried off.
           The game has been replayed after the second leg (0-0)
C1 1972/73 2nd leg won 2-1 by Panathinaikos FC (Athens) (Gre) against CSKA Sofia (Bul) 
           cancelled because the referee ended the shoot-out prematurely after Panathinaikos fourth penalty was saved by Bulgarian goalkeeper.
           The game has been replayed and won 2-0 by CSKA Sofia
C2 1984/85 2nd leg won 3-0 by Celtic (Sco) against SK Rapid Wien (Aut) 
           cancelled due to Rudolf Weinhofer claimed to have been injured by an object.
           The game has been replayed and resulted 0-1
C3 1997/98 2nd leg drawn 2-2 between FC Spartak Moskva (Rus) and FC Sion (Sui)
           had to be replayed because the goals were 8 cm short of the prescribed height.
           The game has been replayed and won 5-1 by Spartak Moskva

Other

Several games have been interrupted for any reason then continued or replayed one or more days after.
C3 2001/02 The match between Rangers FC (Glasgow) (Sco) and FC Anzhi Makhachkala (Rus) played on one game only,
           because of political situation near Makhachkala.
C3 2008/09 The match between FC WIT Georgia Tbilisi (Geo) and FK Austria Wien (Aus) played on one game only,
           because of political situation in Georgia.
C1 2011/12 Trabzonspor (Tur) replaces Fenerbahçe SK (Istanbul) (Tur) in Group Stage
           because of bribery allegations during previous domestic league, after having been
           eliminated at 3rd qualifying round by SL Benfica and having played 1st leg in Europa League
           play-off against Athletic FC
C3 2016/17 The 2nd leg between ŠK Slovan Bratislava (Svk) and FK Partizani (Tirana) (Alb) cancelled after FK Partizani (Tirana) replaced
           KF Skënderbeu (Korçë) in C1 because that team was banned for match-fixing.
C1 and C3 2019-2020 Because of SARS-2 coronavirus, several matchs have been played on one leg only.
C1 and C3 2020-2021 Because of SARS-2 coronavirus, several matchs have been played on one leg only.