Cheating on Chie with Rise in Persona 4 Golden, then having to reject Chie on Valentine's Day. Ouch.
Getting Tali killed in Mass Effect 2 because I couldn't keep her loyalty despite my renegade being almost 100%
Dark side playthrough in KOTOR. You massacre your teammates in front of the Ebon Hawk.
I regretted nothing. DS for life :D
Though Carth deciding to throw himself on Revan's lightsaber was...odd.