What causes an HTTP 405 “invalid method (HTTP verb)” error when POSTing a form to PHP on IIS?

Question

I have one form in a PHP (5.2.9-1) application that causes IIS (Microsoft-IIS/6.0) to throw the following error when POSTed:

The page you are looking

Answer 1

An additional possible cause.

My HTML page had these starting tags:

This was on a page that using the slick jquery slideshow.

I removed the tags and replaced with:

And everything is working again.