Fonts become garbled in many apps

I've come across a bug that has surprised me. There are times when I'm using xfce terminal and something will happen (I'm still not sure what) at which point the fonts in use will become garbled-- line segments will disappear from them, as if the freetype2 library has a serious bug that impacts a font server that is used by many programs.
After the problem starts in the xfce terminal, this symptom then begins to affect other applications, like a disease that is spreading, necessitating that I shut down the system.
Has anyone else seen this? I will take a photo the next time it happens.
I'm running 64-bit Slackware current; I made the ISO about 3 weeks ago.