almost ten years ago, we had the boiler replaced and part of the job involved for some reason, replacing a perfectly good and working wired room stat in the hall. Replaced with a wireless one, mounted over top of the original, but not fixed to the metal back box. I have just upgraded to a Vaillant VCR 407F, mounted at a more accessible height and place, than the original, so to keep things tidy I decided to remove the original stat. and fit a blank in its place.
That wireless stat was fixed to the brick/plaster with 1.5" screws, installer had only drilled the holes a touch over 1" deep, far too shallow - plugs then tightened up. I have just spotted that his tightening up the two fixings, managed to blow a 9x9 section of the plaster off the wall. Only the wallpaper was hiding it, this was not that old, perfectly sound plaster.
On getting home, after the install, I spotted the boiler was half an inch vertically out true and he had forgotten to install a condensate drain. So he had to be called back to finish the job. He still failed to straighten up the boiler, but it was hidden in a cupboard. This, despite my drawing a perfectly vertical line for him, right next to the boiler for him to work with.
This - the same installer who was originally insisting he would have to run a new pipe all round the outside of the house walls, all along the back of the house, then halfway down the side, to the meter - because the 28Kw original boiler's supply was not adequate for the 18Kw one he was there to install. That is until I did the pipe flow calcs for him and proved him very wrong.