Security researcher Michael Jordon has managed to hack a Canon Pixma printer so that it can run the original DOOM
"Canon Pixma wireless printers have a web interface that shows information about the printer, for example the ink levels, which allows for test pages to be printed and for the firmware to be checked for updates."
However, he found that the interface doesn't need any sort of authentication to access. Off the bat the worst anyone could do would be print off hundreds of test pages and use up all of the printer's ink. Jordon found you could do much more, though. The interface lets you trigger the printer to update its firmware. It also lets you change where the printer looks for the firmware update.
In theory, you could create a custom firmware that spies on everything that printer prints, it can even be used as a gateway into the network it's tied into.
To show off what he'd learned Jordon opted to show DOOM running on the printer.
Canon has stated they will provide a fix as soon as possible.