URL A redirects to URL B, which in turn returns an error. The type of redirect, and type of error is unimportant here.
In the past, we would have reported the error observed at the end under URL A. Now, we'll instead report it as URL B. This makes it much easier to diagnose the crawl errors as they're shown in Webmaster Tools. Using tools like cURL or your favorite online server header checker, you can now easily confirm that this error is actually taking place on URL B.
This change may also be visible in the total error counts for some websites. For example, if your site is moving to a new domain, you'll only see these errors for the new domain (assuming the old domain redirects correctly), which might result in noticeable changes in the total error counts for those sites.
Official Google Webmaster Central Blog: Changes in crawl error reporting for redirects: