While my friend was in his Obsidian Nightwing form and he was giving me a lift, I was being continuously disconnected and dismounted when we either left a city or crossed a region. This happened both in Northrend leaving Dalaran and in Eastern Kingdoms. This happened to another guild mate while he was mounted on another person.

edit: I just saw another thread on this topic, sorry about the duplicate I didn't search before hand. You can close or delete the thread.
Edited by Bananimal on 5/14/2013 1:54 PM PDT