When mounted, you can ride into a tunnel two blocks high. This causes the player head to penetrate the tiles at height of three tiles, so you only see deep black while in the tunnel.
Repro:
Mine a tunnel that's two blocks high. Mount a horse. Ride into the tunnel. You should see deep black.
Expected behaviour:
Player should collide with blocks at 3 block height when mounted.
Duplicate of MC-477, please use the search function to see if your bug has already been submitted. Currently over 55% of tickets are being closed as duplicate.