The bug
The interface of a sign remains open when the sign is destroyed. Expected would be that the interface is closed, similar to how an inventory or the crafting table GUI closes when the respective block is broken.
How to reproduce
Place a stone pressure plate with a repeater behind set to the maximum delay
Place a command block with the following command behind the repeater
/setblock ~ ~1 ~ air
Open the world to LAN to make sure placing the sign do not pause the game
Step on the pressure plate and quickly place a sign on the command block
Can confirm for MC 1.12.1.