As a mechanic, I have a few.
I forgot to reinstall the clutch release fork on a 6spd Dodge diesel. It was installed on the TOB but not on the pivot ball in the trans. I didn't realize this until after I pumped the clutch pedal several times and popped the piston out of the brand new slave cyl...which on those trucks is a sealed system and not bleedable (you replace the master, slave, and the line between them as a complete unit). Had to pull the trans again to reinstall. And the 6spd on those diesels is massive to say the least.
I went to bleed the brakes on an older Dodge truck and broke the bleeder valve in the wheel cyl. So I went to remove the wheel cyl and twisted the brake line.
Possibly most embarrassing and most expensive to me...did a maintenance timing belt on a 3.2 Cadillac CTS. Tightened one of the tensioners down hand tight but not fully tight. The tensioner let loose while the boss was on a test drive, causing the engine to stall , leaving my boss stranded on the side of the road, and bending almost all the intake valves on 1 bank.
When you work on dozens of cars every day, you are bound to have a few blunders. These are the only real blunders I can think of.
Last edited by ouch1011; 01-20-2012 at 12:47 AM..