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