You would have encountered a GREAT bug in Facebook “Preview Profile” option. Once you choose the option to view your profile as your friend, you could get access to your friend’s private data including chat, inbox, friends requests and notifications. Facebook has just fixed this bug.
Here is a screenshot of that page. You can see in the image that my profile is been viewed as if my friend is viewing it. From the same page you could access private data of your friend (Not in the image).
Facebook’s response to the bug:
Chat is unavailable as we work quickly to fix a bug reported to us. It should return to normal soon. Because of the bug, people could view friends’ chat messages and friend requests for a limited amount of time if they manipulated the “preview my profile” feature in a specific way. We’ve fixed that issue and took down Chat as soon as we became aware of it. We apologize for the inconvenience.