The following URL will redirect to https://example.com/page.html:
https://secure.wikimedia.org/wikipedia/example.com%5C/..%5Cpage.html
This works because most major browsers interpret \ (%5C) as a forward slash in the URL. The redirect tries to visit this url:
https://example.com\.wikipedia.org/..\page.html
which the browser then interprets as:
The security impact of this bug is minor, but it could be used for misleading users into thinking a malicious link is genuine.