Room 5b

Another lecture theatre at Unseen University mentioned in The Last Continent.

Unlike Room 3b it has a real tangible existence, but the local space-time continuinuum is slightly skewed so that a senior wizard entering the room to take a lecture might discover he is already in there, having already arrived twenty minutes ago. It is safest to cough discreetly before you go in, just in case you are already there.

At least, this is the reason advanced by the Senior Wrangler for not wanting to take lectures in 5b...

The logic might run something like

Well, if I'm already there then it's patently obvious somebody is taking the lecture, so I might as well go back to the Uncommon Room for another sherry, I'm not needed in that case.

Conversely the students might consider:

If I'm already there attending the lecture, then obviously somebody's listening and taking notes, so I might as well go back to Halls and back to bed. Fancy scheduling lectures for first thing in the afternoon...

As with Room 3b, this interpretation of a common space-time paradox is apparently fully satisfactory to both sides involved.