Come across a few this week. Today I saw this. This boiler is a Ferroli Modena 80 E, combi and must be 15 years old. Installed in an airing cupboard, the installer obviously couldn't be bothered to fit the correct flue, so just used the old open flue instead and then used some pipe which went about 1 meter in to the loft as an air intake. Oh and the loft had no ventilation either.
This one isn't so bad, but still the installer needs to take a look at themselves in the mirror.
Worcester Greenstar R28HE, firstly for some reason they used a vertical flue rather than going horizontally out the wall the boiler was fixed to. Strange, but they then used the wrong flue outlet adaptor. The one fitted wouldn't allow the exhaust to slide up over the adaptor, which was causing the POCs to spread in to the air intake side and corrode the adaptor clamp. Because the adaptor wouldn't fit up far enough, some wood screws had been used instead of the correct screws. When I replaced it with the right one, the flue hadn't even been pulled down far enough to sit in the adaptor either. All checked now and it's working fine.