One time, a user phoned up to say that Windows was behaving wierdly, and dialogs would flash up then disappear. Being rather confused by this, I went to have a look. Sure enough, every dialog that appeared just flashed on screen briefly.
After a few minutes of looking for a cause on Windows (slightly hampered by the fact that every time I opened a dialog, it closed, I happened to glance at the keyboard. There was a book resting on the edge of the keyboard, just lightly pressing the Escape key..