The previous owners of this house did so many inexplicable things (some we knew about and others were delightful surprises).
The (upvc) windows looked like they were just dirty. No, actually all the window units at the front of the house we're so blown that the condensation had actually etched the internal surfaces of the double glazing making the glass opaque.
They bricked up the kitchen window, put in a dark wood kitchen with black granite worktops and black tiles on both the floor and walls. All lit with a 15W (not energy saving) lightbulb. The darkness meant that you couldn't properly see that the entire kitchen had been fitted by bodgers. The cupboards we not fastened to the walls properly and the granite worktop had been badly fitted. In particular the hole for the hob had been cut in the wrong place so the hob was fitted right against the back wall (and was a fire hazard). It couldn't be fixed so the worktop was buggered.
They badly fitted ikea wardrobes in 3 of the bedrooms. The ones in the master bedroom had chunks cut out of the doors and sides so that they could fit round the radiator they'd put across the chimney breast (but which was longer than the breast was wide). No problem we thought, we'll just remove the wardrobes (and move the radiator somewhere sensible). Turns out they'd also removed all the plaster behind the wardrobes for not reason whatsoever.
There was a large bit of boxing in that took up a considerable amount of space in one of the bedrooms. We assumed this contained a now redundant hot water tank. No. It was empty except a 21st birthday card and a packet of viagra.
They had redone the bathroom, but the bath was not sealed in any way. So any time you used the shower you got drips through the kitchen ceiling. The bath had been tiled in in such a way that we had to destroy the panel in order to investigate what the problem was. They'd put a pretend plastic sealant strip at the bottom of the tiles, so it wasn't immediately obvious that this was purely decorative (to us or the plumber).
The tiler they'd got to do the bathroom had left a 1" gap in one of the corners with no tiling. They'd hidden this behind a freestanding unit, so it was a surprise when we moved in. There were no spare tiles to finish this off with.
There was a bathroom and a separate toilet, but the toilet was not properly connected up to the soil pipe. What they had instead was a plastic pipe with a dip in the middle that just filled up with shit. To the untrained eye (and to our surveyors) it looked like proper plumbing.
They had decided to convert the garage into a bedroom themselves, badly. Our mortgage valuation had been based on returning it to a garage (and we planned to anyway). It turned out that it was more badly done than we'd thought. They'd simply replaced the garage door with a big upvc window unit, but there was a huge hole underneath this so when we ripped up the floor we found everything was soaking wet and there was a big puddle.
They'd also plasterboarded it , which included covering over all the mains gas pipes leading to the meter. Somehow our surveyor had missed this (despite seeing the meter). The gas people told us this was Not Good. Luckily we were turning it back into a garage.
There's more, but I can't remember. It was a nightmare trying to turn it into a proper house. (It's lovely now).