This is a long story, but hopefully it might stop someone else wasting their time!

At some point during the creation of my latest interior, I realised that the game was crashing on exit every single time. Sure enough, when I removed my new cell, the crashing stopped. Note that this crashing occurred regardless of whether I was in the cell or indeed if I had ever visited it - basically, if the mod was present, the game CTDed on exit.

I decided to try and backtrack to find out what the problem was, by deleting stuff gradually. First the navmesh, then the portals and rooms, then the lights and so on. Now, this is a very busy cell with a lot of items in it, so I started deleting the most numerous items first - in this case wine bottles, of which I suppose the cell actually had about 500. Still no luck. Eventually, when the cell was starting to look very bare indeed, I deleted the 'track lighting' and the crashing stopped.

I restored my cell and removed the track lighting. Back to crashing again! So I decided to try a different approach. The cell consists of one largish room containing most of the stuff and a few smaller ones, so I deleted the large room altogether: crashing. So I restored and deleted the smaller rooms: crashing. Playing around with different combinations of rooms to delete made no difference, but I could eventually stop the crashing after deleting various, seemingly random combinations of items.

I decided to start the whole cell from scratch, and check as I added each new set of items. With the bare walls of my main room created, I found I could add about 50 wine bottles without trouble, but beyond that the crashing started. I then created a new, static wine bottle and created a static collection consisting of my wine rack plus about 50 bottles. I found I could add eight of these to the cell without trouble. However, I do have to have some bottles that the player can take, and I found that as soon as I got to about 50-60 bottles, again, the crashing started.

I was starting to think I'd just have to accept the crashing and put it down as a known bug, then I wondered if changing the file to a master would make a difference. You've guessed it - making it an ESM removed the crashing! I could go back to my original cell and as an ESM there was no trouble.

The upshot is that there seems to be a severe limitation on the number of items you can have in an interior cell in an esp. The odd thing is that during my deletion testing, I was still getting the crashing when the cell was actually very bare, with far fewer items than in most medium-sized interiors.

One good thing has come out of it - I realised that making the static collection was a far better idea than having 500 wine bottles, although the game didn't seem to have any difficulties rendering them all during actual gameplay.

I have noticed that most mods should be an .esm as well. There are alot of very stupid odd issues that arent really caused by anything if your mod is an esp. So if your mod adds new cells and such or anything like that, you want it to be an esm. The only mods I could see being an esp, are ones that maybe add a few weapons, or just some items, but not new places, or people, or worldspaces. All of these things have alot of fun breaking randomly and that can only be fixed by making the mod an esm. So that is the lesson here. If you seem to have issues that arent really caused by anything, make your mod an esm.