Description of the issue:

Occasionally, treated wood fence posts will have additional limbs on sides that are not connected to other fences. This can be seen on the far left and right sides of the following screenshot:

Versions & Modlist:

Minecraft: 1.12.2
Forge: 14.23.5.2838
IE: 0.12-89
Modlist: https://pastebin.com/2kHqhkz6 (From forced crash)

Worth noting that the issue is not solved by breaking and replacing the affected fence blocks. The issue is solved for a few seconds by placing and breaking a fence block on the side with the extra limbs, but the extra limbs will re-appear within a few seconds.

Closing this issue, as testing has revealed that fences added by other mods are affected as well.

Reopening as further testing has revealed that the issue is caused by floodlights.
Turns out fences will attempt to connect to tile.immersiveengineering.fake_light.name, despite the block acting the same as air in nearly every other sense.

Duplicate of #3447, fixed for the next release of IE.