Finally got it to happen again.

It was definitely the /jamba-follow strobeon all from IsBoxer doing it.

I'm rather convinced at this point there is a bug in ISBoxer / IS that sometimes allows modifiers to fall through to the game client that shouldn't.

SHIFT + ALT + F12 is nowhere near any of my keybinds. The only thing I can link it to is jamba master which is CTRL + SHIFT + F12.

I never get the popup that strobing is on, so it isn't that keymap being triggered. I don't have any of those keymaps configured to a key binding. This smells very much like an annoying bug.