They're a plot device almost as old as film and particularly common in movies with a "twist ending". Examples (I'll spoiler just in case even though they're decade old movies):
Sixth Sense
Fight Club
Thirty Rock has used the technique several times, including in last week's hysterical episode when Liz realizes "Oooh, now some of the things Oprah said make more sense..." ("My boyfriend's in 9th grade!" "I fly in first class free because my mom's a flight attendant!" "Well, I did sip champagne at my cousin's wedding...".)
They're flashbacks and they're revisionism, but I'm sure there's some kind of industry jargon for them other than revisionist flashbacks. Anyone know what the actual term is?