The back button on the browser is one of the most used buttons in the browser. It's used far more often then Stop, Refresh and Forward combined. It's incredibly useful for getting back to a recently viewed page or canceling a navigation that you didn't mean to make. Unfortunately some website are punishing users for clicking the back button by displaying error messages.
Why are websites doing this?
Some websites, especially financial websites (paypal and Citizens bank as an example) want to make sure they don't accidentally process a transaction twice. So instead of writing code to check if the transaction has already occurred they cheat. By adding a single line of code they tell the browser not to cache the page, ever. This in conjunction with the POST setting on a form causes the browser to show an error page when the user clicks back.
Why is this bad?
Users should always feel safe clicking the back button. There should never be a fear that clicking the back button could result in an ugly error message.
The actual error message that is shown is misleading and may cause some users to incorrectly change their dial-up or connection settings.
If the users can't click the back button they will need to navigate forward to try to find the appropriate page that they want. Because links don't have to be bi-directional it's possible that they will not find a path back to the page they wanted.
What to do?
I recently had to pay two bills online one through Citizens and one through Bank of America. The interfcae between the two is somewhat similar but the attention to detail on the Bank of America site and the ability to use the back button made a huge difference. Bravo to Bank of America for taking the time to do this right.