(I am sure it is the case, anyway)
Sometimes an enemy tries to reaction fire with an arcing weapon, but can't find trajectory and doesn't fire as a result. This interrupts your soldier movement, which feels strange and gives a kind of advantage to the player.
A save is attached. It is probably a rare case in vanilla, but a regular one on some maps in Piratez or Chronicles.
I could have a look myself, but intuition says that the fix will probably require changing signature of several methods, which wold better be handled by OpenXcom authors.
