if you get a classic minecart intersection ( a T shape) with a curve any cart will either turn 180º (run back) instead of taking the curve or derail depending on your speed or lose any speed it had

how to reproduce

1- make a T shape
2- make sure there's a curve into the T
3- put a cart and push it
3b- put a cart on a powered rail and power then push it
3c- put a cart on a powered rail in a slope and power and then push it
4- see the cart go to the intersection and either lose all the speed, derail or turn around completely