Back in June, the main upstairs bathroom - the one I use - got a new sink. When the plug was raised, it wasn't very high. This caused the water to drain rather slowly.
One night, when I was shaving, I was trying to get the water to drain faster. I was jiggling it, and it stopped moving up and down with the controller. So I had to figure out (with shaving cream all over my face) how that worked. I managed to get it fixed.
As I was planning on getting some Draino to clear out the sink I heard a clunk somewhere in the pipes and the water drained really fast. Cool. Problem solved.
The next morning as I was taking a shower, I noticed the water wasn't leaving the tub at all. It seemed that the blockage in the sink pipes had moved into the tub pipes. I didn't have time to investigate until after work.
When I got home Wendy and Jack told me that they put Draino in the tub. This cleared out the tub pipes, but pushed some black gunk into the tub in their washroom. This blockage seemed to move from one set of pipes to another.
I believe they got that sorted out by now.