Short story:

Game file space filled, the world allowed continued play with no messaging, and the world could not be re-opened until other levels were deleted. This was an offline world.

Long version:

I was playing a multi-day Minecraft session, and I noticed that a gate in the nether that I broke kept returning, although my inventory would collect all the duplicates.

I closed the world, restarted the game, and tried to open the world. I got the message that "there was a problem loading this world"

I then tried to make a backup of my last known good world, to check if opening it corrupted it.  The copy failed, saying I was out of space. I then tried uploading both last known good and the corrupted world to the realm, and both failed. I deleted an earlier world to make room... then tried opening my corrupted world again. It opened. I then quit, and saving took ages. Looking at the file saves then, the world was about 100 megs higher than it was on previous save - when it was only one meg higher. I opened it again to look around and try to figure out what the state of the world was, based on inventory and land marks (I make super tall pillars to mark things, and sometimes giant arrows - a pillar and 2 arrows were missing from view). I saved again, and the 100 meg bloat vanished, so it was nearly the same size as I would have expected. At this point I tried copying it to a realm, and it worked. So now my world, a bit past last known good but still with 6ish hours of loss, is on a realm and I can play it.

Conclusion:

There are glitches around file saves when there are too many worlds on the account, with no messaging that the game is close to some sort of limit. The reason that things I broke in the nether kept reappearing is that the game was moving things in and out of memory when I crossed that border, and its inability to properly save is why when I kept removing the fence around my nether-side castle gate, it kept coming back when I crossed the boundary a couple times, as an experiment.

Suggestions:

If there is a limit on world size / copies of worlds, it needs to be communicated. If a world is at limit during an auto-save, auto-save should fail and the problem should be communicated. Also, ideally, there should be an option to disable file save and allow manual saving, to reduce the number of copies someone may make of a world.