It's known bug, especially with the rocket, but I haven't found anything concerning every type of mount, so there it is:
What you need:
- 2 players. Let's call them A & B.
- Flying mount (easier)
- Water
1. Make both player fly in the air above water (must be 20-30 yard deep water).
2. Make player B /follow on player A.
3. Player A unmount and dive in the water.
4. Player B must /follow player A in the water.
5. Player A mount up in the air.
6. Player B won't be able to follow but instead will spin like the devil.
(Optional) Make player B mount a 2 passenger flying mount and make a Player C mount Player B, for better result^^.
I made a video, but I don't think it is necessary since there is nothing hard to reproduce.
It's not very convenient, but it's fun for about 2-3 min until you have a headache^^.