A while back I was in Berlin, and got lost in Nollendorfplatz station (which is not particularly _big_) for about ten minutes. Now, for various reasons I had been awake for about 36 hours, so mostly blamed it on that, but if this site is correct it confirms my suspicion that that is a weird station. U1 and U3 are on different levels for different directions!

Maybe not common, but not an uncommon design. It is so you can transfer between lines by crossing the platform.

https://en.wikipedia.org/wiki/Cross-platform_interchange https://www.youtube.com/watch?v=E5hXH_mLPzw