HP Laserjet 4000 - Manual feed problem

[thinkpad]

Hi,

In our office we have a HP Laserjet 4000 hooked up as a network printer. Recently, there has been an issue where if a user sends a print job to the manual feed, sometimes, midway through printing that job, another print job which was just a normal print request will come out via the manual feed.

Is this a mechanical error, driver error, or something else?

Thanks.