In custom games, when you try to turn into a ghost bear, the usual stuff happens where you can't hear other players and your gain 2 new abilities. However, when you use it the ghost bear ability, you don't use any ghost "power". The timer for the ghost bear won't go down so unless you get hit out of the form, you'll be stuck as a bear forever. Bears can't attack or roar either, but they can pick up items (can't drop them though).
Not entirely sure what causes this, but the ghost bears will also A-Pose (No animations) the entire time which is funny at least.