When you try to place a Block next to a fence, but its blocked by a Mob (so you can see the Block for a split second and then disappears again because of the Mob stepping onto the same Block-position), the fence connects to the block (even though it isnt placed!)
How to reproduce:
1. Place at least one fence
2. Summon any Mob nearby (i recommend something, that can follow the player. So i used pigs here)
3. Get the Mob follow you (for Example with a carrot in your hand for pigs)
4. try to place a block next to the fence at the moment, when the mob starts walking over this spot
If done correctly, the Block disappears after a split second and the fence ist connected to the air
Duplicate of MC-132428 - please search in the future