The easiest way to replicate this is by trying to place a fence atop another fence while holding a connected lead in your hand. This also happens if you are moving in and out of the block's space and it places for only a second before it registers you're in the way.

What happens is that the fence is "placed" but disappears from both the spot where it was and from your inventory. This leaves an empty spot in your inventory that only gets filled if you manually move an item into that place.

This can cause blocks to be counted improperly in your inventory (displaying the wrong number, numbers changing without picking up/placing blocks, duplicate ghost stacks).