Answered
When I try to close out of Acrobat X the following message appears: "This operation is not allowed, since scanning is in progress".
I was going through the Acrobat X Tutorial and clicked on "File > Create > PDF by Scanning". I know my scanner is not hooked up properly so I was not surprised that nothing was scanned.
I can't figure out how to stop a scan in progress.
If that is too long, you might go to the task manager and kill the Acrobat process(es).