A common problem with the HP1600, HP2600 and Canon LBP5000 series colour printers is cartridge alignment errors.

There are a few different things that cause this problem:

Dirty laser unit.
Faulty cartridges. This causes the calibration patches to be miss-read.
Transfer belt and sensors. Dirt/paper dust build up on belt and sensors may cause calibration patches to be miss-read.
Faulty engine control PCB. This seems to be a common cause for calibration errors and requires PCB replacement in the Canon series.