From GameWatcher: “World of Warcraft: Wrath of The Lich King Classic’s pre-patch is now live and, if you’ve already taken to leveling an Unholy Death Knight, there’s a good chance you’ve noticed the rather annoying ghoul pet bug that’s in the game at the moment.
In essence, Unholy Death Knights have access to the Raise Dead spell in WotLK Classic, which temporarily summons a ghoul pet by default. Picking the Master of Ghouls talent ensures that the ghoul stays by your side until it dies, giving you an undead pal that tags along, chomping on foes. The annoying bug that shipped with the pre-patch, however, makes the ghoul despawn when hopping onto your flying mount and taking to the skies.”
source: gamezpot.com