To reproduce this bug, Find a column of gravel. Hold a tool in your main slot and a torch in the offhand. Destroy the block underneath the gravel or the bottom-most gravel block and quickly place a torch. Rarely, the gravel will just stay on top of the torch and stay there. Most of the time, the gravel will be destroyed as it should be. What should happen is that the gravel would fall onto the torch and destroy itself, leaving gravel items to be picked up. I didn't notice it happening until very recently. I think placing the torch very fast causes this. I took a screenshot and recorded about 20 seconds of gameplay up to when the glitch happened.