URL's that exist, but do not verify

I have a page that lists various urls and I am using verify_exists in my model. This worked splendidly except for one address. I can visit the link in question at the url given to me but each time I enter it into the admin it tells me the url does not ex开发者_运维知识库ist. I had a hundred or so links that all worked except for this one. Does anyone know why this could be happening? Could it be that the actual url is forwarded or something to that affect?

I am not sure about django specifically, but many link checkers do not set a user-agent when they run a link check and many websites block requests with no user agent.
Wikipedia blocks requests with no user-agent, so you can check if that is the problem by trying to create a link to Wikipedia and checking if you get an error.