If the /ride command is used to make a player mount an entity that is not loaded, the command will work however the player's position will not appear to change. Placing/breaking blocks will not work properly, entities cannot be attacked, and no fall or other damage is taken. Pressing shift to dismount does not work, and if another /ride command is used the error says the player is already mounted. Using /ride to dismount usually updates the player to the position of the entity they were riding and restores normal survival functionality, however sometimes the players position does not update and occasionally this crashes the client.

A demonstration is available here: https://www.youtube.com/watch?v=oAkGi1xE_JQ

Steps to recreate

1. Create a new world with cheats enabled but in survival mode
2. /ride @s mount @e[type=sheep,sort=furthest,limit=1]
3. Note that the entity type doesn't matter, this is just an unloaded entity that is likely to exist in a brand new world. If the entity doesn't exist, try again with a cow or other entity. If the player is now riding a sheep because the furthest one away was loaded, dismount and try again until you enter the glitched state.
4. When the glitch is successful, the command feedback will say the player is now riding Sheep but the player's position will not update. Try breaking or placing blocks, damaging entities, or taking damage to see the effects.
5. While glitched, you can try the command again and will see an error saying the player is already riding Sheep.
6. /ride @s dismount
7. The players position will usually (not always) update after this second command and you can confirm that normal functionality has resumed. Sometimes the players position will not update, but otherwise things will be back to normal. Rarely, the client will crash.