In System Preferences under Desktop & Screen Saver → Hot Corners I set up ‘Put Display to Sleep’ at one of the corners.
Occasionally, moving the cursor to that corner results in.. nothing. Sometimes all four corners become inactive in this way; at other times it's only the "display to sleep" corner that is inactive and the others (Launchpad, ..) function normally.
How do I debug a problem like this? What might be the cause? The problem disappears eventually on its own, possibly after closing some app (I use Terminal, Emacs, and Firefox/Chrome almost exclusively, but I have been unable to associate the problem with any application by closing it and hitting a corner).