When I first came across the problem I was running unpatched with no mods, am now running latest patch (1.4.21.0.4) and matters haven't improved. It seems to be an unfixed vanilla problem, but mod list follows for completion's sake:
Spoiler
1HD Weapons on Back TS Edition 1.3
A Quality World Map 8.0
Better Sorting 2.04
Categorized Favorites Menu 0.1.11.2
Complete Crafting Overhaul 1.06
Container Categorization 1.4.20.0
Enhanced Distant Terrain 1.2
Enhanced Night Skyrim Color Galaxy 0.4
Enhanced Night Skyrim High Stars 0.4
Glowing Ore Veins 300 2.00
Horse Fast Dismount 1.0
Immersive Skyrim Thunder V5
Improved NPC Clothing 1.8.1
KenMOD - Time on Loading Screen v5
Kerplunk Watery Rocks 1.0
Killable Children 1.01
Lush Grass 1.3
Realistic Smoke & Embers 1.4
Realistic Water Textures 1.9.5
Shadow Striping Fix 1.3.1
Skyrim Flora Overhaul 1.0.9A
Skyrim HD 1.4
SkyUI 2.1
Smithing Perks Overhaul 1.11
Wars in Skyrim 1.5.1
Weapons & Armor Fixes 1.19
XCE 1.11
Official HD Texture Pack
A Quality World Map 8.0
Better Sorting 2.04
Categorized Favorites Menu 0.1.11.2
Complete Crafting Overhaul 1.06
Container Categorization 1.4.20.0
Enhanced Distant Terrain 1.2
Enhanced Night Skyrim Color Galaxy 0.4
Enhanced Night Skyrim High Stars 0.4
Glowing Ore Veins 300 2.00
Horse Fast Dismount 1.0
Immersive Skyrim Thunder V5
Improved NPC Clothing 1.8.1
KenMOD - Time on Loading Screen v5
Kerplunk Watery Rocks 1.0
Killable Children 1.01
Lush Grass 1.3
Realistic Smoke & Embers 1.4
Realistic Water Textures 1.9.5
Shadow Striping Fix 1.3.1
Skyrim Flora Overhaul 1.0.9A
Skyrim HD 1.4
SkyUI 2.1
Smithing Perks Overhaul 1.11
Wars in Skyrim 1.5.1
Weapons & Armor Fixes 1.19
XCE 1.11
Official HD Texture Pack
http://skyrimforums.org/threads/frustrating-fence-situation.1952/ around https://unofficialskyrimpatch.16bugs.com/projects/7078/bugs/214904 it seems others have had this problem, but I can't find any solutions and it doesn't seem to have been acknowledged as a bug by the Unofficial Patch Project. Has anyone else come across this, or does anyone know how it might be fixed?
