The act of drinking a beer that you did not believe you'd ever tasted before, only to realize at some point while drinking it that you have, in fact, had that beer before. This is not to be confused with drinking a beer blindly and realizing what it is. It is the act of tasting a beer, with full knowledge of what kind it is, but with the belief that it was one you'd never had before.
This happens frequently to beer geeks who either keep poor tasting notes, are forgetful, or who just drink too much beer.
That beer sounds good, let me try that. (sips) Holy beerja vu! I've had this beer before, it's one of my favorites! Wow, how did I forget about this one?