If a beam hits a block, an animation is layed over the block.
When the beam is manually moved while it is still being fired, the animation should not be on the block anymore, once the block isn't hit anymore.
This does not happen, if the beam hits an astronaut(and possibly any entity, which does not consist of blocks.
To reproduce:

1. spawn an NPC, that does not move at all
2. create a wall of blocks behind the NPC
3. fire a beam[preferably a non-damaging beam] at a block on the wall
4. move the cursor to point at the NPC, while still constantly firing the beam.
5. observe as the block-overlay is not removed from the block, that was hit right before the NPC got in the way of the beam.