I'm not totally sure this is a bug but I thought it to be unexpected behaviour and I can't find another reference to it here.
I've just seen the number of suggested edits shown in the top toolbar go into negative figures:
This happened while on the suggested edit queue page, after I had approved a couple of edits. As far as I'm aware I wasn't doing anything different from normal (but then again it's Friday so it's hard to say how much attention I was actually paying).
I have no userscripts/browser extensions that would be interfering with this.