Here are a few of the things that 21 & Over has in common with The Hangover: same team of writers, same “X hours earlier” flashback structure, same “must get X to an important event before time runs out” conceit, and the same emphasis on crazy, drunken revelry. Even in an age where most movies are virtually indistinguishable from each other (i.e. Transformers and Battleship, Clash of the Titans and Prince of Persia, Spider-Man and The Amazing Spider-Man, etc.) that’s a lot of similarities. If I were the copyright owner of The Hangover, I’d probably be on the phone with my lawyers right now.
But wait. The characters in The Hangover were all in their mid-30s, and the characters of 21 & Over are all, as the title implies, in their early 20s. So as you can see, the two movies are actually quite different.