I have a friend that has a legacy Geocities (the MySpace of the 1990s for free web hosting) account (one from who knows how long before GeoCities was bought by Yahoo). The control panel (at geocities.yahoo.com/gcp) won’t allow logging in to his legacy account because it gets stuck in an infinite redirect loop, redirecting right back to itself.
My guess is that the problem has to do with cookies (on Geocities’ servers’ side, not the client’s!), but I didn’t get that far, as I found a roundabout solution to his problem. After logging in, the user can go to http://geocities.yahoo.com/filemanager or http://geocities.yahoo.com/v/fm.html to manage their files. While the rest of the control panel is still not accessible, this was enough of a solution for him.
Reports are that Yahoo refuses to respond about this problem with their servers.