This seem to be due to a hiccup in the rounding of particle coordinates.


Example which I have just caught in a debugger:

Particle initially at 375, 294 (exact coordinates unknown - optimised out)
vx = 0.002009, vy=0.11103

First call to try_move is with nx=375, ny=294; returns 1

Code then goes to update co-ordinates.
new parts[i].x = 375.5, new parts[i].y = 294.10733
nx = 376, ny = 294

Particle moves to a different place than the one try_move was called with.


Edit: This has now (hopefully) been fixed.