I noticed that reviving ghosts (e.g., by using ghostly then resurrect-adv) leaves unit occupancy stuck in the tile. Using launch while mounted removes occupancy from the tile, allowing the tile to be traversed without going prone or swapping places with your horse. There's also a DF bug involving adventurers and minecarts.

Given these issues, we should have a function to recalculate the occupancy of a tile. I think the logic already exists in Units::teleport? A tool to iterate the entire map can be written to identify and clean up occupancy issues.