A pillar of the last throwaway block can sometimes cause a spurious recalculation

1. have only one dirt left
2. pillar up
3. the block is placed, but the player hasn't landed yet so it's still executing the pillar for another tick or two
4. there are no throwaway blocks in the player's inventory, but the current movement requires a throwaway block
5. mark as impossible and recalculate segment

=(

not a huge deal tbh, you only lose like a half second on the recalculation, but it's noticable