When a minecart is launched into the air whilst carrying a boat with two mobs/players in it, the minecart will gradually turn mid air, eventually settling with a 90 degree turn if the minecart is airborne for long enough. This is mostly noticeable when the maximum minecart speed is turned up, but it effects all speeds as far as I can tell.

My setup when discovering it:

The bug in action:

https://youtu.be/zUSbI1X9lEE