Turns out that # is not the best keybinding for "report as spam" in notmuch-search mode, because it's bound to "Print message" in notmuch-show mode. Well, that explains why my emails are coming out of the printer at random times.

The weirdest thing is I can't even remember spending the four and a half hours I believed it would take to make this flakey wifi printer work on a Linux box in the first place, so was exceedingly surprised to have anything coming out of the printer at random or indeed any times