Source: bamsmackpow.com

6. Bringing Bucky Back

The Original Story: During World War II, Bucky Barnes was Captain America's plucky teen sidekick. He took out one or two Nazis, but he was mostly there for moral support.

Toward the tail end of the war, Baron Zemo captured Cap and Bucky, and strapped them to a plane carrying a bomb. Cap got free, and Bucky didn't. He went boom and died. Cap was frozen, and woke up in the 60s, which was only 20 years later, but whatever. Bucky remained exploded.

The Retcon: Except not! Bucky was actually a stealthy murder machine who killed Nazis on a level Cap couldn't. He also survived the explosion, even if his arm didn't. Buck was discovered by the Russians, who turned him into a robot-armed Soviet assassin known as he Winter Soldier.

Bucky returned, fought Cap, turned good, became Cap, died, came back, and now he's off doing whatever. He may be with the Thunderbolts; I don't know, because I stopped reading that series because the art was garbage.