When you have a large tech with silos and active fabricators, activating the build beam is very risky business. First thing that happens is that you get very bad framerate (2-3 fps, vs 30-40 in normal situation) with all the chunks in silos becoming active. The weird thing that happens is that sometimes the build beam effect just remains in place. It does not do anything, it just is there. You can drive the tech around, the beam remains where it was. If you activate build beam again, you'll have two beams of light. So far I have not gotten multiple stuck beams around, but I have not particularly tried either. The extra beam stays there until you reload the game.