most likely it is phased objects that are glitching out.
To explain it better i stole this from wowwiki.
Data on the location of game objects, players, and NPCs is not stored in the World of Warcraft client and must be sent from the server. Phasing, in a sense, decides not to send certain data based on criteria.
As an example, you may have a quest to get the head of a named NPC. After turning in the quest, the server records that it should not send data about that NPC to you. The next time you go there, even if the NPC is still there, you will not be able to see it.
By using combinations of phasing, entire areas can change from grasslands to a city by use of game object phasing and NPC phasing.