There has been many, but this is the worst:
MB 310 diesel – Someone else had done a cylinder head job recently and forgotten to tighten the bolt for the camshaft sprocket. I replaced a few bent valves, timing chain, sprockets and so on, all while muttering over that useless idiot who had left a bolt lose like that.
Got it all together, took it for a test drive, but didn’t get very far. The crankshaft pulley wasn’t tightened, bent valves AGAIN…W-|