Could reveal a few, but I'll put in some tips first..
One: when upside down under the dash of a Morris Minor, and you are drilling into the firewall to mount a mounting bracket for the Sub wiring to the rear of the car, don't just drill straight into the firewall without checking that the battery isn't sitting right behind where you are drilling...
Two: Don't keep telling your son repeatedly not to leave almost-empty plastic bottles of Ice Break coffee flavored milk on the black front carpet of the black interior of his Twin-Cam Corolla Seca SX...let him learn the hard way after his car has sat outside his workplace in the stinking hot QLD summer sun for a full day, whereupon the bottle explodes and deposits smelly crap all over the inside of the front of the cabin...
Three: It is possible for a Valiant Hemi oil filter to work itself loose and spin off if you've forgotten to tighten it properly. Ford and Holden sixes (the old ones) don't seem to do this for some reason, they just leak oil.
Four: Don't wire up a cheap aftermarket ammpmeter straight across the battery terminals, mounting it inside, wiring it up, and then connecting it to the battery last of all, and walking away to get a cold drink, then returning to wonder why your car is full of white smoke...
Five: When a cop pulls you up for not having a bonnet on a car, and he has just demonstrated his monumental lack of mechanical knowledge by pointing straight at the Ford X-flow 4.1 liter straight six, and asking "so what is it, a V8?", don't compound things when he asks "what if something flew off?", by answering "like what?"...it annoys the hell out of him.