Ah, TFM, I remember that! Wait, wait, what just happend...
Not sure if this is a common glitch and my keyboard is trolling me, then just lock it if needed.
I was just waiting for the shaman for building when suddenly I pressed D and my mouse kept going that way.
Panicking, and pressing AS and W, nothing happens...
Wat.

Lag. This has been happening to me occasionally, courtesy of the no-longer-optional 60 fps update.

It could simply be chat lock as well. Make sure you've stopped moving before clicking into the input section of the chat.