I'm surprised no one has brought this one up yet: During the scene at Tycho, starting at 0:52:46 on the DVD version, when the shot is of one of the astronauts (presumably Floyd) approaching the Monolith from the ramp, look carefully at the reflection in the helmet. You can clearly see Stanley Kubrick, with his camera.

It has been suggested that this was not a slip-up at all, that Stanley was simply feeding his ego. This may well be the truth.

~DH