Hi there.
I tried to play with filters like offset, and I have this problem :
I couldn’t undo what I did, and if I delete the offset path, a “ghost” path remains but isn’t visible at export.
More : is there any autosave function enabled ? Because I thought I just had to open the previous version of the file to get back to my orinigal glyph… but I was wrong 
So how “recover” the glyph from the “ghost” path ?
The ghost might be the background layer. You can switch between the foreground and background by choosing “Paths” → “Edit Background” or ⌘B.
Additionally: “Paths” → “Swap with Background” (⌃⌘J) will do what it says: swap the foreground with the background.
The background layer is never exported.
I am not sure about “autosave”, but you can choose to use the Versions-behavior in Glyphs’s preferences (“Glyphs” → “Preferences …” → “User Settings” → “Use Versions”). This will let you revert back to an old version of the file. Personally, I don’t use this feature, so I can not tell you how often it creates a new revision of your file or if this only works with Apple’s Time Machine. Give it a try, it might work for you.
Keeping a backup of your files, like with Time Machine, is a great idea in general anyway ;)
You can copy paste paths from the background to the foreground. There is a “Selection to background” that works also can push stuff from the background to the foreground.
It is there as a short term storage of different shapes. E.g. you like to try a new option for a glyph. You put the current outline in the background that you always have a visual reference to what you have changed. Or you like to compare certain shapes, like the bowl of the d and q. Copy one of the two into the background of the other. There are some tricks that you can do. select one point and press Cmd+Shift+A and the node will be moved on top of the closes node in the background. And you an interpolate both shapes. I use that when I did some changes and like to scale them back a bit.
Wow, nice trick, thanks @GeorgSeifert !
This background option seems actually useful now I know what it is for…
I wish I knew it before 