hi I thought this might be interesting
http://news.sky.com/skynews/Home/Business/Facebook-Shuts-Down-Its-Chat-Feature-After-Software-Glitch-Allows-Private-Conversations-To-Be-Seen/Article/201005115626384?lpos=Business_First_Home_Article_Teaser_Region_1&lid=ARTICLE_15626384_Facebook_Shuts_Down_Its_Chat_Feature_After_Software_Glitch_Allows_Private_Conversations_To_Be_Seen
That was an interesting development especially with the backdrop of the serious concerns over FB and their disregard to privacy. Makes you (or at least me) really think twice about adding a ton of friends for games like Mafia Wars etc.
Glad they fixed it pretty quickly though
I also used to see this while chatting with another person and I thought it was a feature