Oh man... where to begin... Luckily I don't remember ALL 'mess ups'... Here are a few that stick out
The first time I changed my oil, probably first mechanic work I'd ever done for that matter, I pulled the tranny drain plug instead of engine oil drain plug. This was before I had a cell phone, so I had to walk a half mile from my land where I was changing the oil to a house with a phone so I could call my parents and tell em I need some tranny fluid.
I still haven't lived this one down.. I once spent 45 minutes looking for a hole in my tire (spinning it on the hub and spraying it with soapy water) only to have my friend come back out and hold up the air hose right next to me and say, "is this the leak you're hearing?"
When I first got the 4Runner I decided to change the spark plugs and over-torqued one of the dist. bolts, snapping it off. I thought "easy fix, I'll just drill it out"... until my drill slipped and cut a hole NEXT to the bolt.. All the way through the valve cover... I didn't know what to do so I tapped the hole and put a new bolt in. Luckily that one hasn't came back to bite me in the ass... yet...
Fortunately I've had enough 'great saves' to offset the mistakes I've made, at least in my mind.