In The Avengers, during the fight with the aliens from Chitauri before Bruce Banner shows up at the fighting spot, there is a dialogue between Black Widow (Natasha Ramanova) and Hawkeye (Clint Barton).
Captain America: You think you can hold them off?
Hawkeye: Captain, it would be my genuine pleasure.
Black Widow: Just like Budapest all over again!
Hawkeye: You and I remember Budapest very differently.
Is this from Black Widow and Clint Barton's back stories and operations? Do we know what happened at the Budapest operation?