I was just troubleshooting a HTTP Error 404.13 - Not Found error in one of applications that handles some large file uploads. I had set my maxAllowedContentLength to 2097151 (which is the max allowed value for the httpRuntime maxRequestLength parameter, in KB). Oddly, it wasn't working and I was still receiving the 404.13.
Some time later, after actually reading the MSDN documentation, it clicked to me that maxAllowedContentLength is in bytes, not kilobytes! Doh!