HP LaserJet 2100tn
Misfeed description
I had trouble with the both trays of my printer. The problem  was that printer picks up multiple pages. Most times in a multiples sheets document, a rate of about 1 to 10 sheet was misfeeding. The begin of the printing on a sheet and the end on the second ( I never had trouble with more than 2 sheets ) . Rarely, the offset was too important and result in a printing error.
It seems that most hp series printers have paper feed problems. If your printer is older than 3 years, you can expect that kind of trouble.
Tray and manual feed work the same. A  worn up a sheet, and a  avoid multiple feed. In my case I manage to clean the separation pad who was to "slidy". It would have be better changing it but...
As summary you can fix it by yourself with a bit of imagination, buy a reparation kit or send it back to hp technical support. But keep in mind that it's a common problem... ;-)
hp, few years ago, sent repair kit for free but don't do anymore (???)
I found those hp's references (Rx-XXXX-???) can be useful when googling
- RG5-4133-170CN FUSER 220V (NEW)
- RG5-4164-000CN TRANSFER ROLLER
- RB2-2900-000CN PU ROLLER (MP/TRAY 1)
- RB2-2891-000CN PU ROLLER (CASSETTE/TRAY 2)
- RB2-6348-000CN SEPARATION PAD (MP TRAY 1)
- RB9-0695-020CN SEPARATION PAD (MP TRAY 2)