There have been a few questions asking about the error caused by IE 11 not loading SSRS reports...
The solution seems to be to check the query string for blank.gif and make some changes to the URL.
Because only some of our reports are broken I'm asking a more general question, why does SSRS reference blank.gif sometimes and not others? Is there something I can change on the broken reports to make them work now before the website is redeployed with the other accepted solution?