Undo functionality in Gmail is a cool feature that can allow you to take back your decision of sending given message even up to 90 seconds after clicking "Sent". However, be aware that at some very rare occasions it may fail:
This is because when you click "Send" the message is actually sent to Gmail servers and put to send-out queue. And when you click "Undo", it is taken back from that queue. But, because everything happens on-line, when for that very tiny moment when you click "Undo" Gmail cannot communicate with its servers (a half-second moment of lack of access to Internet?) then... puf, there it go!
So, please, don't send a divorce letter or a declaration of war this way.
You may not be able to change your mind... :>