This is a vanilla cell NW of Whiterun.

While looking at the CK for sources of my CTDs in cell -2,2 I looked at some neighboring cells.

Vanilla cell -1,1 has a duplicate navmesh laying on top of the regular navmesh. You can't see this unless you select a cell, and delete it. There is an exact copy right below it throughout the entire cell! (Don't save that CK session if you delete a cell - it would be a dirty edit and unpredictable; just exit w/o save!!)

What effect on the game might that have?

It means that, if Lydia were to glitch under the floor, she'd be under the false impression of being able to move.

That's what I think anyway.

shalani, I've had one reproducible CTD in my mod, ScenicCarriages, and I tracked it down to the vanilla road navmesh. On my PC, the horse would do a 360 turn-around and keep going. For the player who reported it, it crashed Skyrim. I've fixed a lot of vanilla road navmeshes, but if I remember correctly, the problem was overlapping triangles. I cleaned it up and haven't had any problems there since.

I was wondering more about memory issues and ctd impact.
I think the duplicated mesh is right on top of the original, so i wouldn't necessarily expect land-swimming , but who knows- that may be an ai overload and pathing malfunction.

With cell -1,1 in your render window, how many navmesh forms are listed in the cell window on the right side? I only see one loading nothing but Skyrim.esm and Update.esm.

Also, what are the form IDs it lists for the ones you're seeing?

I re-navmeshed the entire cell last night in my mod. When I get home i will fire up vanilla and report.

In vanilla it has one - NavMesh (328 Tris)
VERY Weird - With Skyrim.esm and Update.esm as the only masters, I see no duplicates.

With Skyrim.esm I only see a single copy of the NavMesh in that cell.

Loading Skyrim.esm and an archived copy of my Mod (no re-mesh of -1,1) as active file - and sure enough all the NavMesh is exactly duplicated.
In that copy, it has Navmesh (328 Tris)* (indicating a change, although I haven't touched this cell!)
And right below it Navmesh (328 Tris ISLAND) * (indicating an exact duplicate!)

Cell Form ID is Wilderness* 00009583

Perhaps you simply have a dirty edit?

What's the form ID on the duplicate that's listed as an island? If it doesn't start with 00 then you somehow caused it to duplicate and you should be able to safely delete it by right clicking on that navmesh in the cell window and selecting delete.

It started with 01.
I deleted it - BANG! that was easy

Thanks, Arthmoor.

I will see what effect that has on my game.

still don't know how / why that happened.

I should add that deleting the duplicate navmesh in -1,1 appeared to unsynch all the borders in cell -2,2. all the borders there went yellow from green! I am almost betting that re-finalizing -2,2 to synchronized the borders throws that dupe net back into -1,1.
will report .

You'd just need to finalize -1,1 again to fix that.

Finalize -1,1 even though all its borders are green and none of -2,2 are green?