The game crashes when a ridden entity is trying to ride its own rider.

How to reproduce

First you need an entity that can ride itself, so it needs to have a rideable component that references its own family, i've used the slime as an example:

      "minecraft:rideable": {
        "seat_count": 1,
        "family_types": [
          "slime"
        ],
        "seats": {
          "position": [ 0.0, 1.105, 0.0 ]
        }
      },

Next you can either use the /ride command (1) or give the entity a behavior.find_mount component (2).

Summon two slimes.

(1) If you're using the command, make sure you mark the slimes with different tags so you can select them properly, e.g.

/tag @e[type=slime,c=1] add ride
/tag @e[type=slime,tag=!ride,c=1] add rider

Next, make one slime ride the other. So far everything works as intended.

/ride @e[tag=rider] start_riding @e[tag=ride]

Now, reverse the riding without dismounting the slimes first. The game will freeze and crash shortly thereafter.

/ride @e[tag=ride] start_riding @e[tag=rider]

Interstingly the game doesn't crash if you specify the teleportRule argument as teleport_ride.

(2) If you're using the component, the slimes will path towards each other and as soon as they merge, the game freezes and crashes shortly after.