Unusual issue: a Konica C452 FS-527 finisher tray was stuck in the bottom position, giving the "The output tray has reached its capacity" error. Random searching online provided access to the "super secret admin mode" that the service techs use. If I hadn't watch a copier guy work in this menu before, I would've been totally lost. Here's what appeared to work...
* FINISHER -> PATH-SWITCH/OK and SWITCH OPERATION/OK
I should obviously state that playing around in that menu might break a service contract, if not your printer.