Every once in a while, you might stumble upon a lost link marked as "broken redirect" that seems to be working just fine.
There can be several reasons as to why it can happen. Here are the most frequent ones:
4XX or 5XX status codes. At the time our bot was checking the links within the redirect chain, there was a technical issue, and, at least, one of the URLs returned one of the 4XX or 5XX status codes. Which means that the page wasn't accessible and our bot failed to verify the redirect.
Block. Since some sites might block our bot, one of the URLs within the redirect chain can deny access for our bot. Which results in an unsuccessful crawl and 'broken redirect' issue.
(URL) does not redirect anymore. If our bot finds that the redirecting URL no longer redirects, we will consider this redirect broken.
URL dropped. We drop some pages from our index from time to time to keep it clean and neat. If one of the URLs within the redirect is dropped, the redirect isn't valid anymore for our bot. Read more about Dropped links.
You can open each of the pages within the redirect one by one in Site Explorer with exact URL mode to see if we have them in our index.