I was playing Co-op Arena, Chateau when I completely lost control of my character.
I went over to help a downed ally and for some reason the triangle never appeared. I repeatedly ran around him, smashing triangle until he died, at which point my character stopped registering my controls and slowly walked in a straight line until walking into a walk, where it stayed simply walking in place. I could still control the camera, and use the start/select/PS buttons to enter the menus. But I couldn't move, aim, shoot, jump, etc.
To make things ever more odd the game quit registering the existence of my character. I was still there, but the AI didn't recognize me as hostile and completely ignored me, even when everyone else was dead. This extended to weapon damage. One of my allies come over and attempted to kill my character, hoping that a respawn would counter the glitch. Neither his gun nor grenades registered as hitting me. Same for stray fire from the AI.
I tried turning my controller off then back on, but that resulted in nothing changing. So, after a while of this and the game left at a stalemate with all allies dead and me unable to die, I was left with no choice but to drop out of the game.