When reports of this issue came trickling in, so did speculation as to what caused this behavior. Some offered that the game was ‘self-righting’ after a group of roaming aliens got stuck. This theory sounded plausible. Yet at the time I was working on a theory that aliens did not actually roam. The reason for that theory was that a player's squad could sit on the same spot for 20 turns and never get to see an alien patrol. That theory turned out to be only partially true.
Over a week ago, I hit upon a bug that allowed me to see how alien groups move around. I’m sure others will have seen it as well. It is difficult to replicate but it is not uncommon. This particular bug occurs when an alien group comes – just – inside of the visibility range of one of the player's soldiers, but not close enough to trigger the silly “Oooh, we’ve been spotted, let’s run for cover” event. The ‘fun’ starts when it happens when you’ve just moved a soldier and are able to pull it back. If you do so, you will be able to experience what I have recorded in the video below. While this is just a short recording of a single teleport, I've been able to follow them around the map for several turns on other occasions.
What you see in the video is the disc and his bots teleporting away from where they were before. With it, the game reveals how alien groups move around the map. They don’t actually walk but jump from one predetermined point to another. If their destination leads them to a location close to the player’s position, the game triggers a running animation to make it seem that the group had been roaming/patrolling and hit upon the player’s soldiers.
I have yet to figure out if they follow a set path along the jump points but under normal circumstances, any jump point that the player has ‘revealed’ by getting close to it, is supposed to be taken out of the rotation.
My assumption is that the bug happens when a point is not taken out of rotation, or has been put back in for some reason. In a normal game, the effects of the bug aren’t that severe, but play an Ironman game on Classic and you will likely see your squad wiped out when one or even two sets of aliens teleport right in their midst. Only then you will understand just how destructive the bug is.
One can argue that the jump point system is a rather weak alternative to aliens roam and with end their turns in a cover position. However, that would put considerable strain on the system and lengthen the alien turns considerably. As such, I can understand the choice that Firaxis made in this, but not when bugs like these actually kill the “suspended disbelief” and destroy many hours of gameplay in a game that you're not allowed to save in.