A few times now I've opened a file from an external hard drive, disconnected the drive, and then accidentally select "Save" instead of "Save As." I get an error message that the file can't be saved, but then the document shows "Saving 0%" and I can't do anything to it until it completes. Any time I try to Save As, Close the doc, or Quit Photoshop, I'm told that I can't do so until the document finishes saving (which it won't, even if I reconnect the drive it was expecting to save to.
Luckily I can duplicate the document and save that without losing work, but it's still an annoying bug to encounter.