When Autocorrect fails you or a message arrives that renders the lengthy epistle you just typed irrelevant, you can easily delete entire messages or emails.
Just give your iPhone a shake and the Undo Typing menu will pop up. Select Undo. If you meant to select Cancel and accidentally hit Undo, don't worry. Just give your phone another shake and the Redo Typing menu will pop up.
This also works in Mail if you accidentally delete an email or save it in the wrong folder.
Try this in Safari, Calendar, and Reminders too.