I would say the culprit is the directional force, which takes a deal of calculation in the backgfround. Even though you CUT it out, it still was in the UNDO buffer. This means your memory was still being used, even though you thought you'd removed it! As your memory probably reached it's end, your computer, hiccuped with the dreaded "unknown error has occured." (This is also sometimes called a "memory leak.") I agree with pixelicious, that you should do a 'save as.' It clears the buffer, and should keep your file clear of excess baggage. Also, do a re-boot after your save. That way the computer as a whole is starting out fresh, without an error in its memory. Hope this helps! :>