Im not sure if your aware or not as its not really a "bug" but more a luck thing. If you get a quest that have you mount a drake, and then the quest after that is a "kill" quest, you get stuck as unlike normal mounts, quest mounts have to be dismounted by pressing the button, so you will just walk around while constantly trying to attack.
Edit: Actually, happens no matter what quest follows, may be needed to add a step after the mount quests to "dismount" and then remount your own mount.
Edit: Actually, happens no matter what quest follows, may be needed to add a step after the mount quests to "dismount" and then remount your own mount.