Making a website you with functioning log-in and log-off.

This was the 90's. It was surprisingly hard to implement this in a workable, reliable, secure way, and no one in our company of 50 programmers had ever done such a thing before!

I recall being puzzled for way too long at how to prevent someone from coming to a browser that had just logged off from our web app, and clicking the back button a couple of times to be logged in again.

Now, of course, it is a common and easy-ish task.