Undo suddenly stopped working 2.79b I was working on a project in the VSE and editing some keyframes via the graph editor. I believe I used key: Mirror as my last known command because I was trying to invert the keyframe positions. It didn't work, the keyframe became a solid line and moved downwards for some reason, I went to undo and undo didn't work.

I've spent another 30 minutes in the project thinking it would fix itself, it didn't. I restarted the project still not working.

Looked online, tried shift+alt and shift+w but there was no change or any indication that keyboard change was the cause. I'm using qwerty and it's supposed to be control+z to undo and control+shift+z to redo as its always been. Checking controls it indeed still is. Under undo steps they had been set to 42 a while back and global was always on. I moved them to a lower value, no change. Keyboard is set to the same language its always been in the windows tray. I'm at a loss here on what happened or how to fix it. Only remaining thing I can think of is to mess around with the graph editor and see if that jogs it, perhaps a bug with the mirror options, perhaps it was an earlier operation or a bad strip in the vse?

This is rather upsetting as I still have a fair amount of work left to do on the project, mainly the finishing touches and filters and special effects, and those are the ones you need undo the most for.
Somehow I changed to edit mode in the 3d editor, I don't know how given I was only using the vse, but changing back to object mode fixed the problem.