I'm seeing this behavior on iOS 7 Safari, probably older versions as well though I've not confirmed yet.
We have a web filter. When a web page is blocked, a 302 redirect is issued redirecting to a block page which tells the user that the site is blocked, why it is blocked, and lets them log in to bypass the block. Upon logging in, the user should not longer be redirected to a block page. Revisiting the blocked site, now unblocked, results in the 302 redirect again. The only solution is to clear safari cache and try again, but this is too complicated for the majority of users.