I am not sure how epic of a failure mine amount to, but here goes.
I had a 220D that needed a new water pump. I figured that I was smart enough and there was room enough, how hard can it be? When it is the original water pump and has not been dicked with in 36 years, PLENTY hard. I was able to back out the first bolt. Second was not as accommodating, but decided after much Kroil, to behave. Third bolt seemed to work, until it sheered off. DAMN. In for a penny, in for a pound. Went with the fourth, which came out, but the next sheered off, as did the last. $50 replacement became $600.
Same car, manual shift was messy as could get. I am third owner and prior owners had had it for around a decade each. Got under the car and saw that the bushings for the shift rods wear gone or looked like bits of rubber band any more. Parts is cheap, so I can just stick them on/in where they belong. Under the car, being gentle at first and devolving into a ravenous beast yanking and pushing, pulling and jabbing trying to insert the stupid rubber shits. Ended up I had engaged all the gears and the transmission locked up. I had to snag a used one for a nice cash outlay and tow it to a mechanic to swap out the transmission.
The 220D was my near downfall, but the 450SL did me in. I had cleaned up and played with the instrument cluster in an attempt to get it to seat properly. The original board was cracked and causing erratic behaviour, so I swapped in a good board but moved my gauges over. Seemed to work fine, for a drive, but when I looked down, I saw the temp gauge pegged out. CRAP, so I rapidly return home (three blocks) and shut down. Look under the hood, no overheating damage, so I figure the thermostat is toast and I need a new one. Get the T-stat and begin the disassembly of the housing. Unhooked the battery, and move the alt out of the way. The housing is a no go on coming off. Decide a persuader is needed, so move to larger bits of dimensional lumber to use as impact transmitters with BFH imparting blows. I had a 2x4 up by the firewall and in my smacking it slips, knocks the battery cable, which makes contact with the post, which then shorts out the whole mess, as the alt. has now been pushed into contact with some other thing it should not have touched. Smoke and flames ensue. Fire extinguisher is used, mess is made, car is towed to my Indy, who has to order a new harness from the Father land.
Once that mess is taken care of, he sits me down and advises me that in future I will be charged double should my car come to his shop to rectify my mistakes. I am forbidden to work under the hood. If I do, have to take it to somebody else to unfornicate my mess.