Steps to reproduce: find a lava lake with a flat top edge, drag a belt into the side of that edge, then press r once the cursor turns red and you cannot extend the belt any further, and drag your cursor upwards by one tile
the belt planner will place belts facing upwards (it works vertically too) all along the area where your cursor dragged, despite there not needing to be belts there
see video for details (R is pressed right before the cursor moves up)
