With a computer, correcting your work is completely taken for granted. For example, I've rewritten this paragraph about six times which, twenty years ago, would have required at least two pieces of paper and lots of crossing out (I still remember my Dad's first electronic typewriter which would let you correct the last 15 characters typed -- and at the time we were so impressed!). But the one thing that is still difficult to "undo" is sending an email. (Our friends at VideoJug.com have demonstrated just how hazardous this mistake can be!)
I probably need to "undo" an email two or three times a week, and it's almost always because I've forgotten to actually attach the attachment that I said I was sending... Even more frustrating, I usually remember that I've forgotten about 5 seconds after I've sent the email and I'm forced to send the embarrassing follow-up email: "Ah, yes, attachment actually attached this time...sorry".
A colleague took pity on me and pointed out this tip: set up a rule in Microsoft Outlook to delay sending all your emails for two minutes.
Office Online has all the details on how to set this up: http://office.microsoft.com/en-gb/outlook/HP012328171033.aspx?pid=CH100776981033#2
It took me five minutes to get it sorted and then every time I have an "oh no, stop that email" moment, I just go into my outbox and open up the email in question. Once the email's open, it won't be sent until I click "send" again, so I can take as long as I need to correct it.
The other alternative is to recall your message after it's been sent, but there are a number of limitations with this: both you and the recipients need to be accessing your email from an exchange server; and -- more importantly -- the email needs to still be unread. Better safe than sorry, so get that rule set up today.