The IntegratedStorageBundle throws an exception when a file is missing on the filesystem, when the storage object is passed to the web_image function. This results in an unwanted 500-error on the webpage eventhough only one file is missing somewhere on the page. Of course it is more desirable to ignore the file and show the rest of the page correctly. The second time the page is requested the fallback image is shown. Client specific details are in the comments.
Display the fallback image immediately en eliminate the 500 error.