Minor Description
The driver simulates browser caches by not requesting the images included in a page 40% of the time. In reality, browsers make an 'if-modified-since' request to the server and the server checks if the file was modified later than the date specified in the request and if so it will return the file. Thus, this will incur an additional request/response cycle which we are not capturing in the driver/server interaction.
This bug will require fixes to both the driver and the server side. The driver can use a timestamp equivalent to when the run started. The server code (for PHP) must check for the existence of the if-modified-since flag. For rails, since the app does not service these files, the server will automatically take care of the request.