This has happened to several of my maps recently for no reason whatsoever. It makes my maps unplayable and prevents me from saving new versions of them. I have had to rebuild a map six -Yoinking!- times now thanks to this -Yoink-.
I have lost four maps to this glitch and I now make three or four backups of a map just in case.
FIX IT.
> 2533274907207100;1:
> This has happened to several of my maps recently for no reason whatsoever. It makes my maps unplayable and prevents me from saving new versions of them. I have had to rebuild a map six -Yoinking!- times now thanks to this -Yoink-.
>
> I have lost four maps to this glitch and I now make three or four backups of a map just in case.
>
> FIX IT.
I feel your pain bro! There is a workaround though, so unfortunately you didn’t have to restart your maps. Have a friend load your map in forge by themselves, have them Save As, then use their new copy of the file and the issue will go away.
Howe does that work? I mean, I have the same problem. If I can’t load it in forge, how is anyone else going to be able to do that?
This is a glaring issue that should have been addressed BEFORE forge was released. And it really needs to be fixed, or the forge community will suffer. Sad.
I’m wondering if it’s because your map is too heavy. I have one I keep adding more and more too at a lightmap/fx limit and I’m getting this. On a side note, how many times have you tried to duplicate an Item and you accidentally delete a crucial piece of your map - forcing you to leave Forge and come back???
just me? 
Try cold resetting your Xbox and launch your game again, this happens often to me, don’t worry you are fine.
Often, just be patient. The problem will go away or even visit itself on a different file. Renaming the file under file options, quitting then restarting will sometimes work.
I’m not excusing 343i. This is a very annoying problem for Forgers that’s been around for far too long. They still haven’t officially acknowledged it yet, in any meaningful way that I can find.