I asked by BiL today, what is the worst disaster you've ever had on a job. he went a bit red and told me that when changing a radiator in a very posh house he omitted to put the decorating caps on the TRVs when the rad was off.
he went downstairs did some other stuff and was alerted by the owner shouting about water coming through the ceiling. so, BiL panics and runs back up the stairs and there's water peeing out of the headless TRV. he said that even in a very cold house the temperature had risen above 5 degrees and the TRV had opened and all the water had come out of the tube, under the bedroom floor and into the very densely packed rockwool soundproofing and then into the living room below.
luckily he was working for a firm and not himself so not liable. to cap it all the customer was and still is a well known barrister.