When I start answering a question on superuser, with Firefox 11 on OSX 10.7, then swipe back (two finger swipe using the touchpad on my macbook), I get the following notice. It seems like a javascript-alert. The catch: neither of the buttons work. The only way to get out of there is to kill the browser. I've had this twice today. It seems like a javascript-bug. I cannot close the window, or switch to other tabs, although other windows keep on working.

When I start a new question, this works like it should. I can go back to the previous page, no warning.

When I use the back-button in the browser, it works like expected.

Safari (OSX)

I just tested the same situation with Safari. I should tell you that I have a Macbook with the multitouch touchpad. I do not use the back-button in the browser, but I use a double finger swipe to go back. And that is the cause of the problem. When I use the back-button, the page reacts like expected. The popup appears before leaving the page, and I choose to leave or stay. When I use the swipe, I go back to the previous page, and then the popup appears. If I choose to leave, no problem. If I choose to stay, then the page turns white.