Completed reproducible with Ctrl+alt+del on Windows 7, some other instances also cause this problem, but not easy to reproduce.
Every time this happens, I get the error: “GUI Window tries to begin rendering while something else has not finished rendering! Either you have a recursive OnGUI rendering, or previous OnGUI did not clean up properly.” after a couple of “Skipped frame because GfxDevice is in invalid state (device lost)” messages.
The UI that I have built in the window completely disappears, but the rest of the game is responsive.
Occasionally I can bring back the UI by drawing something new to the canvas, but that’s a 50/50 shot.