Custom taxonomy cause 404 errors with forms

After a lot of swearing through gritted teeth I discovered that, much like the reserved term issue that results in a 404 error (‘page not found’) when processing a form:

http://wordpress.org/support/topic/404-pops-after-custom-form-submission-by-post

http://wordpress.org/support/topic/contact-form-404-error

the same thing happens when using custom taxonomies. In my case the problem was the <select name="taxonomy"> I was using for a drop-down. Change it to <select name="get_taxonomy"> and the form now processes properly.