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There is the failure to file penalty,  failure to pay penalty and the Underpayment penalties and the IRS can apply any or all of them to the same return.   Filing the extention only gets you extra time to file however if you don't pay a required amount with the extension the other penalties can be added. 

 

Read the rules on page 2 of the extension form: https://www.irs.gov/pub/irs-pdf/f4868.pdf

Read the rules on all penalties here :  https://www.irs.gov/payments/penalties