Shortly before 6pm (GMT+1) I somehow fell through the floor at Hawkers Alley. I am unable to replicate it but I did manage to free myself by jumping onto the small fishing boats at Fisherman's Bottom (needless to say some players were startled by my appearance).

I did not test to see how far I could go as I didn't want to break anything.
I think it may have been caused by broken clipping as I walked up a tiny ramp to enter one of the merchant areas, but again I cannot replicate.

Definitely the funniest bug I've encountered so far.