OK after a night's sleep I tried loading the cute files on a few other websites.  The common thread for sites that generated the error was that I had a custom 404 page defined in IIS -> Custom Errors Tab. I found that when I took off the custom 404 page that the error disappeared.

 

I also found that when I put the custom 404 page back on, the error returned.

 

When I left the custom 404 page on, and simply put <% Response.End %> as the first line, the error disappeared again.

 

Can you explain this? Seems that it's hitting the error 404 code somehow...