A Most Unusual Paragraph

This is an unusual paragraph. I’m curious as to just how quickly you can find 
out what is so unusual about it. It looks so ordinary and plain that you would think nothing was wrong with it. In fact, nothing is wrong with it! It is highly unusual though. Study it and think about it, but you still may not find anything odd. But if you work at it a bit, you might find out. Try to do so without any coaching.

This is certainly unusual! 

Source 

This is The Unusual Thing!

What was the unusual? 

Well, you will not find anything wrong grammatically there neither you won't find any spelling mistake in entire paragraph. The letter 'e' is one of the most common letter used in English language. If you carefully observe the letter 'e' never appears in the paragraph.