When falling in a minecart, should a player look up into the sky, and have coords displayed on the screen, the coords will freeze and won't change at all.
How to recreate:
1.Build a tower roughly 50 blocks high
2.Place a rail on top of the tower
3. Place an minecart on the rail
4. Push the minecart off the rail and to the edge of the tower
5. Sprint towards the minecart, while right click it. You should be able to push it off due to collision physics in minecraft, while being inside the minecart.
6. Look up as you fall. You should notice that the coords are not changing.
Make sure to have your coords turned on. And the attached images are my attempt to best recreate the bug in image format. From the 2 images that shown me falling down the tower you can see that the xyz coords remained frozen even though I was falling down fast.
This does not happen to: Boats, Horse and Pigs. Although Horse's fall damage does have another bug, again.