In Safari 3, if you are on a page that contains a form and you have entered text into one of the form elements, you get a warning if you try to close that Safari window. The warning makes you confirm the closing of the window and it can be pretty annoying. If you want to disable this warning so that you can quickly close Safari windows, open a Terminal window and enter the following:

defaults write com.apple.Safari DebugConfirmTossingUnsubmittedFormText NO

You then need to completely close Safari for this change to take effect. If you change your mind and decide that you do want the warnings, you can repeat the command and change the NO to a YES.