If an auto sweeper is in mid-transfer while saving the game, it will then on load drop the item right underneath it instead of completing the transfer.
This is definitely unexpected and should not happen. It can mess up sensible environments, and there's nothing you can even do except loading an older save.
My guess is that the reason is the brief "pseudo power outage" on load that disables the sweeper for one second/tick.
- Have an auto sweeper transfer some items, e.g. between two storage bins.
- Between picking up and delivering, quickly pause the game, so that the sweeper "holds" the item.
- Save the game.
- Load the game.
- Unpause, and watch the sweeper drop the item.