I also found a few likely cheaters in my stint as a TA. they sat together in labs and turned in nearly identical code for every assignment (down to some truly bizarre formatting choices). I didn't have the heart to turn them in since they were failing most of the assignments anyway.
Some cheaters make it impossible not to catch them. We had a guy using his phone during a math class. The professor came by and the kid had covered up his phone with a piece of paper. The phone is obviously not flat and the screen was up and still active, shining right through.
