1. Create a product + node display with some attributes
2. Add a rule/submit handler that redirects to another page (checkout for instance) when user adds an item to cart
3. Add an item to cart
4. Click back button in browser
At this point the ajax on the add to cart form will be broken. What happens is ajax_get_form() can't find the posted form build id in the cache.
From what I can tell this is really a bug in the Drupal AJAX framework but I wanted to get some consensus here first. The back button problem only happens when there is a form submit redirect - if you simply click a link off of the add to cart page and then hit back then there is no problem.
I'd say resolving this is fairly important because having redirects working properly is crucial to any client that needs a "cartless" experience.