I was recently (er, a few minutes ago) monkeying with some software and jacked up my user profile. I knew it jacked up my user profile because I could login as other users on my computer, always an important step in troubleshooting… Anywho, turned out it was an item to mount my TimeMachine that my account was trying to start automatically at login. I had just drug it into the Login Options in System Preferences like a simpleton. The offending file to fix it:
~/Library/Preferences/loginwindow.plist
Why this file isn’t com.apple.loginwindow.plist has always made me wonder, but alas it isn’t. You could theoretically just remove the offending part of the file, but it’s not exactly formatted as you would expect.