DH and my first flat together was a grotty rental. We did quite a lot of work - painted, put up shelves, new toilet seats etc etc. We were very proud. One day the agent came round with a builder, saying that there was a water leak from somewhere in our building into the shop below and could they see if it was coming from us. Of course, of course they could. They looked around, couldn't see anything, went away.
They came back a week or two later, leak was still happening, could they look again. Still couldn't find anything but resealed around the bath.
Then again, a couple of weeks later, still happening. Faffed around with the kitchen sink. Presumably this was going on at the other six flats in the building as well.
This carried on for a while, until one day, just the builder came by, he'd been sent to find the leak and not give up until he had. He looked at the shelves we'd put up and got a funny look on his face, asked if they were there when we moved in. Noooooo, I said.
Well, said the builder, by the looks of that bit there, there might be a pipe running behind that wall. Better have a look.... so down come the shelves, and they opened the wall...
I still have a photo of the waste water pipe for possibly the entire building with six lovely holes drilled straight into it. DH was mortified.