The only truth that sticks

Hey there wise and all-knowing Friends List, I'm hoping to tap your collective knowledge a bit. I'm looking for examples of betrayal in the TV shows we know and at least some of us love. I'm interested in all kinds, from the ones we're in on it from the beginning (Faith joining forces with the Mayor), the ones that blindsided us (the character who'd been helping Gormogon in Bones), and the ones where the betrayals are perpetrated by the good guys (Wiseguy.) Gimme what you got.

A note about spoilers: I'm pretty lackadaisical about spoilers, but I try to be courteous in general towards others. That being said, please remember the recent hiatus in my TV watching. I would prefer it if, for instance, all BSG after the mid-season break was avoided (I don't know that there's any particular reason I might need to make that request, I'm just hedging my bets).

Oh, if anyone wants to talk about examples from Spooks/MI-5 that would be great. The only complete series I've seen is the most recent one, the 7th, but I'd definitely be curious about how that issue has come up in the show.