If you setup SharePoint to use a STS for authentication, you may have an infinite loop problem like I did :) Here was my scenario:
I have a custom STS, and I setup a trust relation between this STS and SharePoint. When I try to login to a site, as expected, the browser is forwarded to STS login page. I typed in my username and password, then browser started redirection to SharePoint and then again back to STS. This was an infinite loop, and it was weird that Internet Explorer does not complain when there is an infinite loop.
Anyway, when I googled this situation, first link was: Setting the Login Token Expiration Correctly for SharePoint 2010 SAML Claims Users. I applied the suggestions which basically were changing the default timeouts. However this did not fix the problem.
I started thinking about the scenarios that could cause this problem. I know from the logs that STS login worked perfectly ok, and it created claims, and sent it to SharePoint. SharePoint was probably refusing the claims, and sending the user back to STS, STS was redirecting to SharePoint again (as there was a session cookie after then authentication) … The problem was when I was setting up the claims, I had a typo, and SharePoint was expecting another claim from STS, and hence causing the infinite loop.
I fixed the claim typo problem, and tried it again, nope again infinite loop. with the help of my one of my colleagues, we found the problem. When I click the site from IIS manager (click browse link next to the site), IIS is using localhost address, and this was causing the infinite loop. However when I type in the real address (not the localhost one), everything was working as expected :)