Problem occurs in both Windows & Linux. After a certain period of time of the server being up, when you right click and NPC it will produce a WoW Error message. However, after a while it can also make your entire WoW client crash and the problem is unfixable until you restart the entire server. This happens to EVERYONE on the server when the interact with the NPC, not just certain people.
Screenshot of WoW Error: http://i42.tinypic.com/i69ytx.png
It's always this same error, the "cmath.cpp" error.