Gym lockout is one of the biggest problem which deters active gym battle. Ever since Pokémon Go was launched trainers are experiencing errors when entering gyms. What makes the matter worse is a lockout system which prevent trainers from participating in a battle when the error is first encountered.
In Pokémon Go, gym battle is the core feature for multiplayer gameplay, at least until the real PvP battle is released. Some trainers might have encountered a Level 1 trainer with the name “Trainer” stuck in a gym.