I've been suspicious that a certain application that has been doing network access on their UI thread. (This is a horrible practice and anybody should be embarrassed doing it). I tried to bring up some properties that I knew resulted in a ping to a server and the UI hung.
I waited. No UI. Still waited. No UI.
And then I turned off the network connection, which would cause network routines to return an immediate failure instead of hang indefinitely or wait for a very long timeout. (It's kind of like issuing a ThreadAbort to break out of an infinite loop). And the UI immediately sprang back to life.
Caught red handed!