I've had a few good ones.. I was installing some OEM euro led tail lights on my 2008 GTI, which requires a few wires to be run, and some coding changes on the ecu, and also replacing an intake pipe. So, I run my wires, connect them, hook up my laptop and make the coding changes, lights work great and all that. I go replace the rear intake pipe, since I had been sent a replacement after the finish got clouded up from the heat (they used the wrong clear on the first batch). I get it all buttoned up, and go to start the car, and it turns over and over, then barely catches, and will only idle at like 400rpm. It's running like ****, and won't take any throttle without stalling out. I take the intake back off, and it looks fine, but I toss my old one back on, and it's the same. I hook up my laptop and scan for codes with the vag-com software, and I see something about a pressure sensor error. I ask a guy online who is a VW tech, and he says that sensor is inside the ecu, and it isn't replaceable.. ****! I fried something in the ecu when I was goofing with the wiring!
I call and have the car towed to the dealer to have them look at it, since it isn't something I can fix, and I'll just have to pay and get it replaced. Well, they put it up on a lift, and one tech noticed the intercooler hose was collapsing when it was barely idling. They start popping things apart, and come to find out, there was a plastic bag inside that intake pipe that I replaced from when they packed it to ship out. I had looked, but it was like 3ft long and had bends at both ends, so I didn't see anything. It sucked the bag through the pipe, down into the compressor housing on my turbo, and was lodged about 1/4" from the compressor wheel. The tech pulled it out, and the car was 100% fine.
I felt like a real ******* paying that $150 bill at the dealer. LOL
I also had another really stupid one last weekend on the same car. I replaced the turbo with a larger one, did new plugs, coilpacks, etc etc. I had a hell of a time getting the downpipe and stuff off because the bolts are hard to get to, and were rusted pretty badly. I had to take the passenger wheel off, the inner wheel well cover, and the axle / ball joint so I could get enough room to get to the stuck bolts. I get it all done (after taking 3x as long as it should have because of the disassembly), and I pull out of the garage, and the car feels weird, like I have a lumpy tire. I go around the block, and it's really feeling odd, so I pull it back onto the lift at the house. I only took the passenger wheel, axle, and ball joint, so I go loosen and retighten all that again. I go back out, same thing.. driving really weird again. I pull back in, and my dad is like did you check the lug bolts? I said yeah, I only took this wheel off, and you saw me put it back on and torque it twice. He goes and checks the drivers side... ****** lug bolts are finger tight!
I think I just hit those lugs with the impact while I was over there, figuring I was taking all the wheels off to swap to my summers, and then forgot. I torqued those, and everything was great until I blew the intercooler hose off at 27lbs of boost hahahah
) Well a few days went by before I put the blade back on... after three pulls.."Why Wont this thing start? Its always started easy!" Well some dummy pulled the plug wire when he pulled the blade, and forgot! 
DOH!!