I like this question. Should get some interesting responses.
For me I have a few.
Necaxa, Vasca da Gama 2000 - Club World Championship
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1-2 vs Zenit 2008 - Super Cup
Back when we were still on top of the English game it seemed we didn't appreciate how important these 'Charity Shield type' competitions were in persistently peddling our being perceived as a super team across the world; how that can affect future stars decisions on which club to go to, which clubs they view as the best.
Yes we dropped out of the FA cup in 99/00 to enter the Club World Championship (as it was called), but we went there woefully under-prepared and outclassed.
https://www.bbc.co.uk/sport/football/20660585. Look at Real's media drive when they win the Super Cup, the Club World Cup, the old Intercontinental Cup. The effect is cumulative.
Add in our performances throughout the best part of a decade and we're left with youngsters the world over thinking City amongst others are the clubs to go to. You can imagine the discussions between foreign fathers and sons;
Father: "Son, Manchester United are probably the biggest club on earth, they...."
Son: "*roflmao*, I've never seen them win anything. Their football is horrible. And but, but...City...Pep...De Bruyne...Aguero...PSG...Neymar...Mbappe..."
Now Liverpool's recent success should have them joining the pack prising future top talent away from us; starting with Sancho if today's article proves true:
https://www.skysports.com/football/...-sanchos-next-club-kaveh-solhekol-the-insider
1-1 vs Celta 2017 - Europa League
The pathetic way we approached and ended this game filled me with disgust. Also see.....
0-0 vs Sevilla 2018 - Europa League
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1-2 vs Sevilla 2018 - Europa League
Another disjointed dour Mourinho approach to both legs. These performances plastered proof far and wide for all to see how far we had fallen.
1-1 vs Liverpool 2019 - League
The way Ole changed the formation twice, single handedly handing Liverpool the ascendancy in a game they were choked out of with our initial approach. For 65+ minutes we were full value for the three points and for breaking, early in the season, Liverpool's now ridiculous run. This game had all the feeling of another invincibles stopper. What that loss could have done for Liverpool's mentality at a time when City were closer... Also, despite my tactical grievances, all we had to do was close down the cross.
There's probably more but these were the ones that popped straight back into my mind. The Nani red card is definitely still a sore point. But when I consider than injustice I just end up thinking about Drogba vs Ovrebo, and I'm alright again.