This is a problem with twitter, gmail, everything (including a few apps I’ve developed :).
So you go to twitter.com. The page appears but still seems to be loading something in the background. You click on the username field and type the username, tab-out to the password field but just as you are typing the password the page loads completely and the focus is set back on username field. Before you understand what has happened and react, half of the password gets typed in the username field. This does not happen very often, but when it does its embarrassing especially if someone is sitting besides you.
I imagine the solution should be simple. Set focus to “username” field on load, but don’t set it if