I’ve been getting it lately in the worst possible moments, like misplaced dragging on a curve. Oops, Cmd+Z? Nope, enjoy your wacky outline! I know it’s very difficult to reproduce, I just wonder what a good report would be here? Save immediately, describe the last operation and send file? Or is the save state before that more useful?
What is the app version you are using (Glyphs → About Glyphs)?
3534 and 4108, always the latest available cutting edge version.
Do you see the undo issue about the same in both?
I haven’t been using G4 much yet, it’s 95% G3
OK, good to know. These undo issues are difficult to track down. If you experience one, let us know about the actions you took just before the issue appeared. These issues are also frequently caused by plug-ins, like Sync Metrics Keys.
I do have some plugins, not that particular one though. Just happened again, was using Tim Ahrens’ Paste in Background sccript (which is having issues in G3 and not functional in G4 for me @TimAhrens ), selected paths, used Tim’s Mask to Master → Undo was removed for this glyph but should be functional again. So now I have to dig out the outlines from a previous file, very annoying.
Edit: Actually undoing any action after running Paste to Background seems to trigger it. G3534
Seems like you are using outdated versions of my scripts? “Mask to master” was re-named a while ago.
I know, I just call it that out of habit. But I reinstalled the scripts and now it works in G3, maybe something was stuck before. Still doesn’t work in G4 and I use it all the time, so it’s a huge hit to my workflow 
The “Paste Background” script should work now in Glyphs 3 and 4 (and potentially in Glyphs 2, still).
Sorry for not being clear, it’s “Insert Glyph to Background” that doesn’t work in G4.
Ran into this repeatedly (but not one after another) just scaling paths in G3 3534. Overscaled, wanted to undo, nope. I would prefer not to punch my screen, but I was really tempted to.