I have had that happen to our HP OfficeJet v40. i was just sitting there, and this printer spit out an english report that i typed up like 6 months earlier, and it wasnt even hooked up, just power, no phone, or usb.
its trippy, i know some of the more recent printers have ram, and in some cases flash memory, but why would it just randomly print it? electromagnetic interference? or maybe its just a fault in the programming in which after an "x" amount of time it is programmed to clear the cache in the printer, but some how it was executed incorrectly and printed it instead.
just some food for thought.
but yea, i dont think thats its a big deal, but if it does it repeatedly and starts wasting your ink/paper then have it replaced. and if it happens to all users of the printer (at least 70%
then there might be either a recall, or a firmware update for it (i highly doubt it, but if its possible, then they would do it because its way cheaper to install a bit of code than to replace an entire printer)