back arrow behavior varies from browser to browser. And a lot of it is simply outside out control.
In practice the back arrow is designed for web sites more than web apps, and the best solution is simply not to use it. Provide the user with sufficient navigation options, and train then to stay off the back button.
Actually the browser pretty much trains them already. Given that it doesn't actually work.
As to why you get the error? Well the back button simulates the previous non-ajax request, before this one.Utimately you can look in the log to see what the request was (and there often isn't one) but it's not likely to be what the user actually expected.
Some browsers attempt to get the page from the cache, but we specifically prevent the data from being cached, so it's not in the cache, which is why the browser generates the message.
cheers
Bruce