I use Spaces a lot and get this happening about once a week; windows vanish never to be seen again, placing windows when in “View All Spaces” mode is weird because they seem to jump away from where you’ve dropped them.

I’ll try this tonight.

macosxhints.com – 10.5: Kill the Dock to possibly resolve issues with Spaces

I was having major problems with Spaces today, and I didn’t want to reboot my machine because I had a ton of tabs open in Safari. Spaces would not respond to Command-Tab or switching apps with the Dock. I was only able to activate Spaces with my hot key.

I looked through the processes, and there was nothing whose name contained spaces. I turned off Spaces in System Preferences and turned it back on. That didn’t help either. I even ran the Spaces.app, but there is no way to force quit it. SO I went out on a limb and tried an old reliable method for fixing Mac window weirdness: I killed the Dock’s process, causing it to restart. Viola! Spaces works fine now.

Apparently Spaces is merely a new feature of the OS X Dock. So if you find yourself in this situation, here’s what to do:

1. Launch Terminal, in Applications » Utilities.
2. Type killall Dock.

The Dock will restart, and you’ll be (hopefully) back in business with Spaces.

[robg adds: I had this happen last week — I sent some windows to a number of spaces, and they just vanished. Each app’s Window menu showed the windows, but they wouldn’t activate. The Dock’s contextual menus did the same. In Spaces’ F8 mode, the windows were gone. Killing the Dock brought all the windows back (into space 1, but at least they came back). Note that the “cleanest” way to restart the Dock is to logout and login; alternatively, you can also kill it in Activity Monitor without using Terminal.]