Something weird just happened to my fluxbox display. Any explanations?

I've used fluxbox for years and this is the first time I've seen anything like this. It started with a blocky distortion in the digital clock which I have on the right side of my panel.Then the captions on the taskbar became distorted. Each time I changed them by switching windows it got worse. So I exited fluxbox. By this time the fluxbox menu was affected too but I didn't see anything odd inside any of the windows. When I typed startx again, everything came up normal.

Hopefully it won't happen again but I'd be interested to know the mechanism. If it's an X thing, why did it only affect the window manager and not any of the other programs?

PS: now that I come to think about it, I once saw something rather similar after an AntiX dist-upgrade. I had Conky on my desktop and it started blocking out like that. Anticapitalista provided something that fixed it but I've forgotton what it was.