I rode a horse into water, it worked fine until it got 3 blocks deep, then I fell off as expected, but even when the horse came back to 1 deep water, right-clicking would only jerk the camera angle, not get on the horse.
After that whenever I tried to place a block, it would only work if there was a previously destroyed block in that same space, and would turn back into that block the moment I placed any block there.
Mobs also seemed to act normal, except it was impossible to interact with them, and they didn't notice me at all.
It fixed itself when I /killed myself, but I didn't try to reproduce the bug again.
Everything was the same when I switched between Creative and Survival, except in Survival I got the block I placed back.