So how did you cancel it? I assume killing the nc process wasn't enough because it had buffered the whole file, so you had to do it at the printer?

Happens all the time when something goes wrong, like malformed files, wrong driver loaded, etc. All printers have an internal buffer, so when one goes crazy printing pages of gibberish, the only solution is to kill the job directly on the printer, or turn it off asap, then go to the computer and empty the print queue should something exist there as well, then do the same at the print server, if there is one. The point is that once a print is started, all buffer will eventually end up to the printer, so it's the 1st device we must stop and the last one to restart once we're sure no data to print is left between the application and the paper, that is, not before all spoolers have been emptied.