The bug

Using the /summon command to summon a turtle with a passenger (Illagers) with the Passengers tag while the turtle's body interacts with watch causes the game to crash.

How to reproduce

1. Summon turtle with the Passengers tag. (Passenger should only be illagers.)

/summon turtle ~ ~ ~ {Passengers:[{id:vindicator}]} 

2. Place water

3. The turtle is "trying" to swim in water, thus making the game crash.