OK so here's an issue I came across today...
HP Laserjet 1022. At times it prints fine, others the print will be blurred down one half of the page.
Changed the cartridge and it starts printing fine, so leave it with the user who calls after five minutes to say it's blurred again.
So I print a self test page and it prints fine, then a Windows test page and it prints fine, then I stand back and get the user to print a document which is blurred. Hmmmm.
Remain stood back and get the user to print a Windows test page, then a printer test page - both blurred. Then I move forward and as I do so notice that my shadow has moved, blocking the sunlight which had been shining directly on the vent on the side of the printer, all of a sudden it's printing OK again. Move back out of the way of ther sunlight and it's blurred!!
So obviously some of the light was falling through the vent and onto the mirror causing the issue. I was really scratching my head with it for a while until I noticed the sunlight!! A carefully positioned ring binder has solved the issue.