I think it was against Dino Bravo in the late 80s/early 90's. Bravo knocked Bret off the apron and into the guardrail (which was a spot that Bret would do quite frequently, so it was only a matter of time before he got himself injured doing it) and then Bravo further aggravated the injury by kicking on Bret when he was down on the outside (clearly being unaware that he was really hurt.)
Am I the only one not grossed out by the pic Adam posted? I'm sure it hurt like hell but it isn't hard for me to look at at all.