When attempting to generate an impostor that leads to a failure, a failure message appears. If the user revisits the avatar's page a few moments later, the failure message disappears and acts as if the avatar is still attempting to generate impostors.
The "Generating Impostors" button remains stuck in its "in progress" state indefinitely, preventing the user from trying to generate another impostor.
This seems to happen consistently, and resolves itself after a few hours.