I visit my ASHX file, and it outputs a PDF perfectly. If I visit the very same ASHX with a different query string (I append DateTime.Now.Ticks to the end each visit), and I get this error:

Server cannot append hader after HTTP headers have been sent.

My code is below:

        copy.CloseStream = false;
        document.Close();

        var r = context.Response;
        r.ExpiresAbsolute = DateTime.Now;
        r.BufferOutput = true;
        r.ContentType = "application/pdf";
        r.AppendHeader("Content-Type", r.ContentType);
        r.AppendHeader("Content-disposition", "inline; filename=" + context.Server.UrlEncode(formType.File_Name));

        r.BinaryWrite(copyStream.ToArray());
        r.StatusCode = 200;
        r.End();
        originalReader.Close();
        copy.CloseStream = true;
        copy.Close();

There is no other place in this code that headers are sent. You are seeing the entire interaction with the Response object.

I've tried to use r.Flush(); and r.End(); I've also tried not sending them if they are already there, but this causes other issues.