Why does my RepRap freeze mid print?

Recently I was setting up my new mac laptop so that I could use it as a 3D printing host, I installed slic3r and pronterface, finishing up the configuration with a couple of smaller test prints. Happy days! Everything working perfectly. So I started a much larger print job, leaving it do its thing. I came back about half an hour later to see that the RepRap had “frozen”, it was still hot, but the printhead wasn’t moving. Just frozen in space, mocking me as a big plastic lump oozed out of the hotend. Why did it do that?

This is a companion discussion topic for the original entry at https://reprage.com/post/30132499768/why-does-my-reprap-freeze-mid-print