I have noticed that after adding a variable product to the cart, if the user immediately clicks on the browser back button the application can "appear to freeze" and "empty the cart" (as reported by end users).
My specific observations: the cart summary at the top of the page returns to the prior values; the Add to Cart button appears, but clicking on it or any other control does nothing, however clicking on a menu allows the app to continue. In fact the cart contents are accurate (nothing is lost) but the old summary is being displayed after using the back button. In my testing this happens with Safari (both OSX and iOS), but not Chrome (Windows or iOS).
To experience this behavior visit a test site http://k9nwsource.com/test/ using Safari.