10. Discovering Nick Fury Had Wiped His Memory
Uncovering that you've had your mind-wiped is a pretty difficult matter regardless of how you look at it. Discovering you've been mind-wiped, working out why, and then confronting the terrifyingly resourceful person who did it is an entirely more difficult matter, and yet Captain America pulls it all off with aplomb at the closing of the Secret War comic.
Steve manages to uncover that he and several other superheroes were recruited by Nick Fury in order to arrest Lucia von Bardas, the prime minister of Latveria - in a plan that actually involved them secretly overthrowing the country, killing untold numbers of Latverians, and then having their memories of the event wiped before any her could get too furious about them not being told about the true awful nature of their mission.
The discovery of this act leaves Fury a full-time war criminal, and in the bad books of the vast majority of the superhero community - who would have otherwise been totally unaware, if not for Cap's uncanny ability to realise when he's had his memory toyed with.