http://www.youtube.com/watch?v=RlNBVOcuW3M
Pretty shocking. I'm a little bit.... taken aback. The problem is that Bethesda seemed to have ignored this MAJOR glitch in their mod tools. Why????
Incidentally, just in case there are actually even a handful of people left who believe Todd Howard's claim that Creation Engine was "rewritten from the ground up" - (when in reality it's simply an optimised Gamebryo), please consider that in a supposedly rewritten engine all bugs from the previous engine magically reoccur. Curious.

(Unless they fix/change that)
They had fixed that for FO3 with the advent of ONAM lists in ESM flagged plugins' headers, listing all cell children (NAVM, REFR, ACHR, ACRE, PGRE, ACHR, PMIS). So far, Update.ESM has no ONAM list even though http://i.imgur.com/kzuTI.png. I've an ESM file with gobs overrides to placed stuff (interior and exterior) and it works flawlessly regardless of extension or ESM flagging, albeit I've no NAVM edits in it yet.