If you have a spark shell running, and you forget it's there and start another one, you'll see errors like the ones below.
If you're like me, all the useful bits that point straight at the problem have scrolled off your screen, so you'll probably waste some time on google looking for other people having problems with error: not found: value spark. Hopefully that google search has lead you here, and you can now go kill all your extra spark shell sessions.
