404 errors on a 301'd page

[EAOM]

I current have a site that when run though a site map tool (screaming frog or xenu) returns a 404 error on a number of pages

The pages are indexed in Google and when visited they do 301 to the correct page? why would the sitemap tool be giving me a different result? is it not reading the page correctly?

[CodyWheeler]

Run it through HTTP Live Headers in Firefox (it's an addon). Just search for HTTP Live Headers. That will give you some insights as to how the browser is reading your page.

It's very useful for when you need to diagnose issues like this.