New app losing your signin?
At the weekend I had a problem with a new site where it would keep asking me to sign in every time I visited. It's well known now that ASP.NET 2.0 changed the way that timeouts worked, so I checked the web.config to make sure a long timeout was specified for the forms authentication. Check.
But, still, after a very short period of time when I returned to the site I'd be Mr. Anonymous again. 
I also noticed that sometimes I'd see an error: Unable to validate data
It took a few minutes to google up some suggestions (it seems a long while since this particular problem was an issue for me) - but Jeremy hit the nail on the head. And now that I know what the problem is, I'm not sure I did come across this version before..
In a nutshell; this new site was just being visited occasionally by me - and IIS on that server was setup to stop the app pool after 30 minutes of idle time. And the default behavior is to generate a new machineKey whenever the app starts - which invalidates the auth ticket.
A quick fix - put a static machineKey into your web.config. Don't forget to make it a randomly generated one though, eh? 