Steps to Reproduce:

1. Place two mounts (horses, boats, minecarts, etc) close together;
2. Mount on one of them;
3. Switch to the other directly from the one you're currently riding.

Observed Results:
After dismounting, every other mount in the world only works once, and the one you rode first no longer works. The only way to get them to work again is to reload the world.

Expected Results:
You can swap mounts freely, and use any of them as many times as you want.

Notes: Manually dismounting before switching seats will avoid this issue.