Something that I encounter on a fairly regular basis is a “drop ceiling” that has gotten wet due to a leaky roof. Upon arrival one usually sees something like this:
Drop ceilings have their uses, but if the roof has a leak, usually the tiles are hopelessly destroyed. The tannin in the leaves on the roof gets in the mix and it makes this nasty brown stain.
In this case (rotated 90 degrees now), it was more than just pulling down the damaged tiles, we also had a light fixture in the mix:
At first, I thought maybe I could cut a hole in a replacement tile and somehow fit it over the fixture, but it quickly became obvious that it just would not work. So I had to take down the fixture:
And here, tile around the light replaced, and fixture put back: 
And then a box of all new matching tiles arrived, here it is all done:
