Mine.. Rookies just join our company, this rookie have lot of talent. One day said rookie followed me to big clients. To fix some expensive machine, after some check up, and insane amount of stress we managed to find the faulty parts and replaced it. When everything is up and time to last check up. Said machine didn't turn on, no matter what we do it just stay silent.. Turns out the rookies forgot to plug one connector. 6 hours of work was like an illusion to us. I was like HOW DO YOU F THAT UP!!!. Well that was mine how about you?
One time when me and my collegue were fixing a machine a client bought from us, we took a week dismantling the thing, and looking everywhere for the malfunction, after we found the issue, got a new piece and batched the whole thing up, same with Jii-san the damn thing won't run or work, as silent as the freaking silent-hills; we took another week re-opening and double checking everything was ok, turn out what, THE FREAKING POWER CABLE WASN'T REATTACHED AS HE FORGOT TO PUT IT BACK WHERE IT WAS. We just wanted to commit suicide by slapping our heads in blocks of tofus when we found out.
I once spent two weeks debugging a code I wrote in two days. It was not until I was done that realized how incredibly stupid I was. But that is how you get past being a rookie... by creating a long list of things to never do again.
Worse part is i wore it the whole day as i didn't notice what was wrong. My brothers were holding their laugher as they saw the funniest sht in their entire life while my classmates also did the same. I didn't notice it at first until the teacher said that i'm breaking the rules...
Spending 3h straigh mixing chemicals and hitting every mark possible (heat, pressure, water, acid, metals etc) but measuring the weight of, guess what - SALT (NaCl) WRONG!!
Me and a Co-Worker. I was a practician at the time, installed a new Washer for clothes. We forgot to pull away the Travel anchor which prevents the core from wobbling left and right during travel. As a result, the washer broke by going left and right. *Cough* 500 euro down the drain. Woops.