Nice thread.
My best one by miles: a second or third round in the hardest cup I ever played (old system where you could face managers from 10 levels above yours). I faced a lev+1 manager who happend to be an old "enemy". I had played him several times in previous seasons for as many bitter defeats. Very active, full squad of 7 stars + SA (who appeared as 8 stars to me). First leg away, he put the game at midnight or 1 am, can't remember. I convinced myself to try and attend the game. Only to face an internet crash! I could reconnect after 6 minutes to witness the disaster in the making: 0-4. I made a rapid couple of changes just in time to score a last minute goal: 1-4, I'm gonna die with honor!
The day for the secong leg I had to play a vital league game against the leader. I was thinking of fielding my B team for the cup when a little voice told me: play it as if were going to get through! changed my tactic at the last second and there we go. I played defensive and the first 20 mins my team was hopeless. And suddenly, out of nowhere, there came a goal from my DC (the only one he ever scored I think). Half time, I was roughly equal on shots when my AM gave me the wisest advice of his life (did not last - rest assured!): let's go all attack ! I waited for 20 more minutes and then, could not resist: switch to hard attacking ! 80 mins, counter attack and there comes the second goal. He became nervous, made some changes. Me too: I fielded my 3 subs on minute 85. But nothing happened. Well, at least until last minute when I got a FK. For the sweetest goal ever from my 3rd sub (not even a FK expert). This season, I managed to reach the final only to lose on PK against a team of similar quality as the one I faced in this memorable game. I felt it was utterly unfair but this game (and this whole cup campaign) remains as my favourite anyway.