This is another 'magic' particle ordering issue. Particles with an ID higher than PCLN get an extra update compared to ones with a lower ID. The first time you create the laser, you most likely won't get the issue, but if you save with photons on screen, you risk them leaving particle ID holes in the lower numbers, which get filled in by PCLN and move differently. You can watch the two different streams in debug (hit "D") and see which stream has the lower/higher # values.

has no breaks (aside from initial load when newt gravity gets set)