I have yet to figure out what I am doing (if anything) or what sites I'm visiting to cause this, but sometimes vimperator goes into a trance and stops listening for commands. It appears to still be running since the status bar is still there and updates the title when I change tabs. And it only seems to affect one window, not all of them.
How can I wake it up and get it to start working again? I'm not doing anything crazy like hitting ignore-all-keys (S-ESC) or ignore-key (i). The sure fire, but painful, way to fix this is to restart Firefox. A couple of times I've had some tabs that wanted me to save something before quitting, and if I clicked the Stay On Page button and didn't actually quit Firefox, that was somehow enough to get vimperator to start listening again.
I've seen this question asked a couple of times on #vimperator on Freenode, but have yet to see an answer.
EDIT: Another workaround that I've found recently is to attempt to close the problem window (but have the "Warn me when closing multiple tabs" options set) and then click "Cancel" when asked if you really want to close multiple tabs. I'm still looking for why vimperator gets into this situation and how to get out of it in the future.