Rovas, thanks for your reply. The site navigation is pretty open, almost a matrix style navigation. There is a left hand navigation menu and plenty of links on various pages. So I don't see any potential in throwing an invalid page flow exceptions for my site - except when I'm in a linear progression and what I need from session is not there as in the case of previous bookmark. This has merit in catching bad flows from two or more browsers, but if the flow is not a bad one, I can still be in trouble with different values for the same session key. On browser X, he might now be on page A with color="blue" in session and on browser Y he might be on page A with color="brown" in session. In actuality, there is only one color in session for both browser X and Y and it is either blue or brown depending on which was saved last.
One thing that did come to mind, was if I was able to replace all links with buttons. That could eliminate multiple browsers opening through the links, but I'd still have the problem if they did a File->Open->New Window.