Arkham City: I was trying to grapple to a building that hadn't rendered properly, and once Batman flew through the building, I was right underneath Arkham City and gliding gaily through the air infinitely.
AC Brotherhood: Same as above, but I fell through the ground ended up "swimming" underneath Rome. There was no water, just the animation of Ezio swimming through water.
Mass Effect 3: Two of them; on the Embassies level on the Citadel, there's an asari standing next to a bench. If you run into her (Hold A), you climb up the bench and fall down to the ground floor of the embassies. Nothing but Mario Kart 64-style sprites that rotate with the player rotating the camera, but if you walk a certain direction, you eventually teleport into the floor below the nightclub on the Citadel.
After I had installed the Extended Cut, the game went a little weird. Once I had selected my squad and went to the weapon menu, the game went "CRITICAL MISSION FAILURE", forcing me to repick my squad and select weapons. But this time, their character models were facing the other way or were a lot more raised than usual. Then I reset the game and it all went back to normal.
Fallout 3: Shot a raider in the face with a shotgun in VATS and watched his head blow up. When I went into the Super-Duper-Mart and got what I wanted from the place, I exited the shopping centre and saw the very same raider I shot in the face... but just his face. Hard to explain, but his eyes, teeth, and brainmatter were intact, assembled, and floating in the air, making it look like someone just shot the skin off his face and only the guts remained. I'll be damned if it wasn't fucking hilarious though.
I also had a fun little glitch when I was replaying InFamous yesterday. So, I was in the Warren, completing a sidequest when I see a Trashman. He hadn't seen me yet so I ran up behind him with my Giga Blades and hit him, he flew forward right into a trash heap, where he got glitched out and was spinning and bouncing to and fro and making that noise that they make when you hit them, repeatedly. It was quite amusing.
