It may also get stuck behind the main window (like dialogue boxes often do so). If you have your main window NOT maximized (but don't collapse it to the task bar), you can move it around and see if it's behind.

You know...? I do KNOW that... I think my brain went on vacation. Thanks for stating the "usually" obvious! Good grief...

You might be able to assign a different shortcut for the 'cancel operation' thing which would mean youd be able to put it closer to the spacebar, which is select arrow by default.