I believe facebook is keep on “enhancing” how they check the access token.
Today, I have received an error message with content
Error: OAuthException: Error validating access token: Session has expired at unix time 1304348400. The current unix time is 1304348963.
There is nothing wrong with that error and in fact, I am happy to see that! At that time, the session is left opened for a long time without any activity.
In the past, I simply get a general error message telling me something is wrong with the access token. But now, I can tell the session has expired! This helps the support in case users report this error to me.