@derobert Hmm, I forget. It was relatively recently. And I think you announced it here. Before I had got to it. But it was fixed quickly. What package was that?
That's a kernel bug, Debian bug #789037 aka upstream bug #99161. It was introduced in a recent kernel update, and you probably saw it after the reboot (to install RAM) because you're now running that kernel.
The fix is already available; you need to install it (and reboot).