On plugging the printer back in later in the day it was dead as a dodo.

IIRC HP have a "secret" counter in the machine that will cause it to
"break" after a certain amount of use - it's possible that this may
extend to failing after a certain lifespan.

Don't know what happened over the old invoice appearing, but another
secret "feature" the use of tiny dots on the page, so as to provide an
audit trail for anyone who might want to know the serial number, date
and time of a print to perhaps defeat faked documents. Colour printers
tend to use yellow dots, which appear black under a blue light.

http://www.eff.org/issues/printers
http://w2.eff.org/Privacy/printers/docucolor/