After editing my keyboard shortcuts, I found that after using the Quick Filter and then deactivating it again, all FC's keyboard shortcuts had been cleared, including not only the ones I had redefined, but even the key mappings that I had left at their default settings -- i.e. I now had no keyboard shortcuts at all.

The error was easy to reproduce, because I thought I had accidentally cleared the shortcuts myself, so I restored the default shortcuts in the Define Shortcuts dialog, made sure they had been correctly saved and were working, then tried the Quick Filter operation once more. The default shortcuts vanished again!

I guess I should have backed up my settings, but I certainly didn't expect this kind of behavior from an otherwise excellent program.