Registering a 3pid can cause you to be kicked out of a room

[richvdh]

And worse, if you were the only one in the room, you will be unable to rejoin.

Steps to repeat:

1. Be the only user in a room.
2. Send a 3pid invite to the room, to an unregistered 3pid.
3. Register the 3pid yourself.
4. Observe that you are no longer a member of the room; rather you have an invite to that room which you cannot accept ("no known servers", or, if the room's extremities get cleared by delete_old_current_state_events, "No create event in auth_events").

Until #7804 landed, you wouldn't have been able to reject it, either. That much at least seems to work now.

[richvdh]

Until #7804 landed, you wouldn't have been able to reject it, either. That much at least seems to work now.

actually I suspect this is incorrect. As long as the room hasn't been cleared out by delete_old_current_state_events, you can still reject the invite. If the room is cleared out by delete_old_current_state_events, you get a stuck invite, as per #7139.