When dismounting from a "vehicle", the game isn't checking the right place if any coordinate is negative.

Way to witness the bug

• Create a new single-player superflat game.
• Head to South-East quadrant ( /tp 10 10 10 )
• Lay a rail
• Put a minecart on it
• Enter the minecart and then shift-exit it
• Destroy the block you were put on (optional - fill it with lava)
• repeat the enter-exit cycle
• Notice you have been ejected to another (safe) block

• Head to North-West quadrant ( /tp -10 10 -10 )
• Lay a rail
• Put a minecart on it
• Enter the minecart and then shift-exit it
• Destroy the block you were put on (optional - fill it with lava)
• repeat the enter-exit cycle
• Notice you have been ejected to the same place (optional - you are now swimming into lava)

Second part can be also done on NE or SW quadrants, to the same result.