How could I forget, two more involving my very favorite old truck...
When your TBI-equipped chevy truck starts cycling between running good and almost stalling in 20-30 second cycles, and you remove the bed to replace the fuel pump, remember to replace the piece of 3/8" diameter 1.5" long rubber hose that goes from the top of the fuel pump to the rest of the fuel line. Because if you don't, and you put it all back together, the truck will drive you nuts trying to figure out why you STILL only have 5 PSI of fuel pressure.
When you get that nice random misfire telling you that you need new plug wires, and you replace them... don't assume you didn't get a brand new set of BAD plug wires from the parts store.
And one a friend has recently experienced - a mid-1990's chevrolet 4.3 V6 DOES have a distributor on it, no matter what your dingbat "mechanic" buddy told you. And when that distributor cap is cracked, no manner of fuel injector cleaning, plug wire replacement, spark plug replacement, or ignition coil replacement will make the truck run right. And lastly, even though the distributor cab has numbers on it for the plug wires, if your buddy is used to working on Mopar V6's, and he attempts to transfer that knowledge to a GM V6, it's going to run like absolute garbage because Chrysler and GM apparently number their cylinders differently.
Also, straightening that kind of mess out, in ten minutes, in the dark after work will elevate one to god-like status in the eyes of some, and they will expect you to know how to fix anything.
So sometimes it's better to act dumb.