Back in the day
In my first job out of university in 1978, I was a member of the team which developed the Dasher D200, Data General's low cost, microprocessor (MC6802) based serial terminal. I wrote the assembly code which scanned the keyboard.
As this was a microprocessor-based product with a display, the bright minds on the team were looking for interesting things to do with it. We added a pair of control sequences which would allow you to download and execute code (downloaded as S1/S9 ASCII hex files) from display memory. This was done for our own amusement, and made it into the release ROMs. Of course, malicious hacking was far in the future and we gave not a thought to the fact that this feature could be misused in any way.
Now that we had the ability to run random code on the processor, we needed some "tests". One of these was an implementation of Pacman, another was Space Invaders. We tried (and failed) to make either of these games fit into the amount of display memory on the final product (our prototype units had 2x the standard amount of display memory, so the games fit easily). If we had been able to fit the either of the games in standard display memory, the terminals would have shipped with the code stored in an unused part of system ROM, accessible with some secret keystroke sequence. It was only because we were a few bytes too big for display memory that this didn't happen.