ones that stand out to me was losing a template to drill out tap hole on a bath, so measure the distance on the taps and drill it out. then picked up the tap and looked at the chrome elbows which kicked out and made the holes 20mm to close. No way of getting around it, needed a new bath.
Tipping the Trap water back down the sink is an absolute classic.
Worst of all mistakes was leaving a gas pipe poking out a wall on set of flats ready to be run around to the meter. The clever builder thought it was an old one (despite it being shiny) and cut it off and rendered it in. So had to chip around it but wouldnt get the block of concrete from the centre of the pipe, so thought it would be clever to compress the gas pipework (to 3 bar) and open the lever valve and shoot it out. Unfortunely the pipe was facing my car on a road about 20 yards away. So compressed it up and fired away regardless, then i heard a shout... so i ran out the front and luckly the tiler was standing on the road on his phone where a lump of concrete had hit him on back of his head, he was alright about it after i explained but he thought id shot a paintball gun at his head.
At least it didnt hit my car though, just had a new windscreen.