When I was just derping around with my mobspawner in SMP, my inventory "locked",(1 as I could not obtain items from it without the items going back into place.) Also, I could not hit any entity while this was happening. The exception is- I could mine blocks, as well as pick them up. They would be useless, however, because (see 1).

What was supposed to happen was that I would be able to play minecraft normally? I don't really know...

What actually happened was my inventory "locked" and I couldn't hit anything.

Steps to Reproduce:
1. This bug happens randomly to me only in SMP, so it is probably not a local world bug- probably is a server-client bug