Boats crash/break and can kill their passengers when falling certain distances

Full text of the comment from migrated 9/11/2020, 1:38:27 PM:

I saw the same Reddit post and decided to investigate. I skimmed MCP 1.12 because I still have it lying around and that version is affected.

In what it calls EntityBoat’s updateFallState, if the boat’s status is not ON_LAND, fall damage is nullified. That will be the case when falling from most heights, because the last time the boat’s status was updated was before it hit the ground. There is only a small exception because the boat checks blocks up to a thousandth of a block below it when updating its status. If it ends up just barely above the ground, its status becomes ON_LAND before hitting the ground, allowing fall damage to take effect.

This happens at the given heights because gravity subtracts 0.04F from the boat’s Y velocity. If it were exactly 0.04, the boat would have fallen an integral number of blocks every 2 ticks out of 25: 12, 13; 49, 51; 111, 114; 198, 202; and so on. The boat ends up falling very slightly less than that because 0.04F is rounded down as a 32-bit float, placing it just above the ground and allowing it to become ON_LAND before hitting it.