Avril and Katy being robbed BIG time.
Disturbia not winning Best Dance Recording because it lost to a SEVEN YEAR OLD SONG. If the song wasn't nominated back when it was released, then you shouldn't nominate seven years later.
Adele winning a Grammy for a live performance when all the other nominees were studio recordings. Not fair at all.
Random Access Memories winning over Red

Skrillex winning over better quality dance songs.
Songs that nobody knows winning over songs that everyone knows
Use Somebody actually winning something

fun., Bon Iver and Zac Brown Band winning best new artist

Hunter Hayes being snubbed.
Amy Winehouse being rewarded for her scandalous behavior when she swept with Rehab

Wrecking Ball not getting a nomination.