I also have the recaller widget on my desktop and this seems to happen when I hang up from a phone call (at first it seems to only be when the call waiting display showed up but I just had it happen with a normal call).

When I add the widgets back they go right to the correct place, including widgets like Dr Launch where i have more than one instance of the widget (1 per desktop).

It's annoying to have to add them all again but I'm getting good at it (and the better I get at it, the more annoyed I get at having to do it)