This one was pretty beautiful, and fun to experience!
We were doing quests in Terokkar Forest..
The quest was:
The Big Bone Worm (Group) - http://wow.allakhazam.com/db/quest.html?wquest=10930
This is a questchain, consist of 3 parts.. We were doing the 3rd one(where the big worm pops out/ (LV 67 Warrior, LV 69 Priest and LV 64 Rogue/me..), when our Priest somehow triggered a BUG! His mana-regeneration gone wild! No matter how many Greater Heal did he cast, his mana was full again, by the time his casting was finished. He couldn't even cast anything that fast that it would have made his mana under 95%!
We thought we do some instance just the 3 of us to gain crazy XP, but when he entered the instance the BUG became untriggered :((( So if you experience something like this, better not try to get into instance, die, or anything..Just be happy and kill anything you see :))
Im posting this because i hope there is someone out there the with the time, patience, will and energie to investigate and tell us how did it happen..
Was it a one time only BUG? Or is it possible to trigger anytime? If so, it would be an insane fast lv-up tactic or farm tactic for all the lucky Priests out there 
Cant tell much about details, but feel free to ask if any info could be useful.. (what he was wearing etc..)
Good Luck!