HTML domain validation failed

What kind of issues can cause the HTML domain validation to fail?

I have uploaded the file exactly as required, and can see the number at the URL exactly as the validation page lists it. But trying to verify the domain just says it has failed and I should try the CNAME method.

How can such a simple validation technique fail? The site is not redirecting the URL or anything - I've tried that. There are no CDNs or anything fancy on the account.

I don't have direct access to the domain yet to do a CNAME validation.

Any clues what to do next?

--- Jason