I have a python script that pings 12 pages on someExampleSite.com every 3 minutes. It's been working for a couple months but today I started receiving 404 errors for 6 of the pages every time it runs. 
So I tried going to those urls on the pc that the script is running on and they load fine in Chrome and Safari. I've also tried changing the user agent string the script is using and that also didn't change anything. Also I tried removing the ['If-Modified-Since'] header which also didn't change anything. 
Why would the server be sending my script a 404 for these 6 pages but on that same computer I can load them in Chrome and Safari just fine? (I made sure to do a hard refresh in Chrome and Safari and they still loaded)
I'm using urllib2 to make the request.
So I figured out what the problem was. 
The website is returning an erroneous response code for these 6 pages. Even though it's returning a 404, it's also returning the web page. Chrome and Safari seem to ignore the response code and display the page anyways, my script aborts on the 404.
Troubleshooting 404 received by python script