When in a Video Chat or Visit, 2 users can join. If a third person attempts to join, or if a user attempts to rejoin quickly after leaving, they will receive a notice that reads “a 3rd party attempted to join”. In the event this is caused by a user leaving and rejoining quickly, and they are a valid participant of the visit, it can cause confusion on what to do next.
Typically this can be resolved by waiting a minute or two and rejoining, or by clearing cache.