When a minecart traveling along a rail track changes direction, any player or entity in the cart does not change the direction it is facing to match. I believe this is intended behavior for the player, but for other entities this seems like an unintended consequence.

Specific example: Using an armor stand in a minecart to simulate having a sentry patrolling around a perimeter, that armor stand will always face the direction it was facing when it entered the minecart.