This occurs because the game simply issues the command to Revert one time, and if Terra has an opportunity to act on it but cannot, the command will fail and the game won't make any further attempts to repeal the Trance status. Terra will thus remain in esper form for the rest of the battle until she is killed or the player reverts back through the Revert command manually. It should be noted that her ATB gauge will act normally without any side effects. This bug was fixed for the Advance release when Terra is inflicted with the three statuses mentioned above, but the bug can still be achieved with the Quick spell.