Early film cameras used hand-cranks to advance the celluloid in the camera, and even though there were plenty of good operators, there was still a unique look to the resulting footage. There would usually be slight variations in the frame rate, which would give you faster/slower motion as well as changes in the exposure. You also get a lesser version of this effect at the beginning and end of a take with film.

Source: Give Your Footage a Hand-Cranked Film Look with This Prolost After Effects Plugin