I was just out of school and my first job screw up ( yes even I made a mistake ) left hand thread issue, did an oil change and broke the sump plug off brand new car with 500 miles on the clock ( boss wasn't impressed )[:-]
edit: the boss/owner was my dad so I got a thick ear[:@]

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I know its an old car now but it was new when I started ( he who makes no mistakes makes nothing ) worth remembering because we are human

 

My first screw up was crossthreading a sparkplug on a 90something toyota Mark II. Luckily I had time to fix it before the owner returned.