Unless it happens in a real game we are not likely to look into this
Guess what:
1. Load the save.
2. In the lower right corner of the screen you will see biters and spitters pressing themselves against the cliff.
3. Observe some of them occasionally teleport to the other side.
I have some memory of a report similar to this where biters would try to attack the player and when they got close they would all "bounce" away. Something about the tightly-packed logic of biters decides that it should "spread out" the packed biters and does it similar to this.
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The issue is that the “bounce” speed is given by how close they are together, so if they're really close, they'll get bumped really fast. I limited it to the normal unit speed, which fixes it in this particular case. I'm sure within 5 minutes someone will show up with a mod that changes biters max speed to the speed of light and say it happens again, and I'll move that one to Won't Fix.
I'm sure within 5 minutes someone will show up with a mod that changes biters max speed to the speed of light and say it happens again, and I'll move that one to Won't Fix.
This is good to see. I didn't think to report it before, but reading the OP reminded me I had seen similar behavior on a test map in map editor where I used cliffs to block off access to some stuff on 3 sides, forcing the biters to go around (effectively funneling them at the defenses I was testing), but occasionally I was seeing some bypass the cliffs (some groups were having trouble getting around the line of cliffs I had set up and so were massing up against the cliffs).
My Mods: Classic Factorio Basic Oil Processing | Sulfur Production from Oils | Wood to Oil Processing | Infinite Resources - Normal Yield | Tree Saplings (Redux) | Alien Biomes Tweaked | Restrictions on Artificial Tiles | New Gear Girl & HR Graphics
This may be the problem I was experiencing when a biter nest mysteriously appeared on the wrong side of a cliff once. I still think though, that this was the nest appearing on the other side of the cliff when the biter created it.