Not sure if this is just a coincidence, but I was running around a spawned North Qeynos just now, and doing /slash commands to see what I could do. I noticed on my servers World.exe window that another client had connected to Queen's Island (different zone than I am in). After that, /who all would no longer work. Now, no /slash commands at all are working.
While writing this, I tried /shout HAIL! and the client crashed. Just the generic "want to launch VS2005" dialog.
I'll verify if this is caused by a second client connecting or not today. Like I said, could be coincidence.

Yah it must have been something else. I just logged 2 other clients in from different PCs, one over a VLAN, and it seems to be fine. The symptoms are that no more /shash commands will work, and eventually if you keep moving around, the client crashes.
Cannot reproduce at this time. Feel free to note and ignore.

This happens when the encryption between your client and the server gets desynced. The server/client can no longer communicate properly and eventually the server will send the client something that will crash it. Was this before the ghost delete problem that I fixed a couple of days ago? Or was this after you updated within the last day?

I think it was after the last update. I had 2 other clients connected at the time (actual players, not me) when I noticed it. I'll watch for it again. Always looking for a reproduce.