Pacus can flop over terrain that can't support them, such as open air. Their falling behavior is triggered on a different "phase" from normal movement, so on high game speed and high game load, they will take a while to start falling down, Looney Tunes style. Except they keep flopping around during that time and can "jump" over as much as 2 tiles of air. Pausing will force them to fall.
They can also flop through closed (set to auto) vertical pneumatic doors.
Both behaviors shown on screenshots: first screenshot shows pacus getting to ladder over a tile of open air, the second one shows a pacu flopping through door. The door wasn't opened any time recently, it's certainly not pacu managing to sneak in while a dupe opened it.
Steps to Reproduce
1. Add enough CPU load that the game chugs on fastest game speed. 2. Place a bunch of pacus on land. 3. Surround them with doors on one side and air on the other. 4. Set game speed to max. 5. Watch physics being defied.
This issue has been fixed in either a future version or updated in a previous version.