Error when connecting to database server during install; you find that when attempting to add a database server for a new farm the wizard refuses to take the domain login credentials which you enter, even though those credentials have full rights to the SQL dbase and server.
CHECK: who are you logged in as? Are you in fact logged into the application server, or the server where you are attempting to install SharePoint, as a local administrator?! Log out and log back in as the domain administrator or log in as the account under which you are attempting to connect back to the database server and do the farm install. Of course, if this isn't the case, then you need to go back and check your login account's privileges on the database server.
Reason: you've probably been at it all day long, doing lots of installs and builds, along with testing, and you need to take that break you were planning on taking 2 hours ago!
You tell a colleague about what you did just so you feel better and confirm that anyone can make mistakes. Your colleague laughs and says he's going to blog about it so the whole world knows that you made a silly mistake :-)
Your Site Actions button stops working, along with other buttons on your SharePoint Web forms, like create buttons.
Yours truly, we're only human.