Ever had a bad day with your composed email. when you wanted to send it to someone or type in wrong subject forgot to attach a file and by unknowingly clicked “SEND”. GMAIL team has come up with a solution to save you from such situations by giving you a chance to UNDO the sent email.
Google Labs is providing an option of giving an Undo link after you click on the send button. But the catch is You have got 5 seconds to undo it else its gone.
To read more about this feature and its usage read here @ Official Gmail Blog.
Hope it saves you some time