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.
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We're tracking this issue in MCPE-133774, so this ticket is being resolved and linked as a duplicate.
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