Attempting to ride another players "tamed" horse causes a game crash, much like a penalty boot.

Put the summary of the bug you're having here

What I expected to happen was...:
should be warned or just not be able to mount the "tamed" horse.
Alternatively you could get bucked off the horse every time.

What actually happened was...:
game shut down,

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here

2. Get a friend to tame a horse..(this time the horse did have a saddle, and amour),
also I did have it happen on "13w24a" with an unsaddled "tamed" horse.

3. try to ride your friends "tamed" horse