My near fatal mistake happened while changing out the water pump on my 95 e320.
As some of you know, the zinc-plated-bolts on MB's are relatively "soft" and will strip rather easily, which was what happened to me. I had failed to insert the allen deep enough into the bolt and stripped 3 of them before I figured out to switch tools.
I finally succeded by switching to a 6mm allen-deep-1/4"socket wrench to get the job done.
The only thing that has come close to that type of DEEP sinking feeling after a stripping a bolt was standing at the alter ready to say "I do." The warm burning sensation that methodically rises from yous knees to your throat, all the while breaking you into a cold sweat...