When a minecart is in motion and a player in spectator gamemode stand in front of the minecart, the minecart stop moving.

Spectators shouldn't be able to interact with entities but they do with minecarts.

1. Place a minecart
2. set it in motion (like with powered rails)
3. go in spectator gamemode
4. set the player in spectator mode in front of the minecart