Using the /ride command will cause the game to crash. When Zombie Bob rides Rob Zomb and I call Rob to ride Bob, the game crashes.
First step => /ride @e[name=Bob] start_riding @e[name=Rob]
Second step => /ride @e[name=Rob] start_riding @e[name=Bob]
Third step => Game crash.

The consequence of the game crash is a cycle. => StackOverflowException.