016: Don't trust the government.
We recently got hit with a fine from the IRS for late payment of some Hide and Creep taxes. Not that we actually paid late -- Stacey sent the check at the appropriate time. But the US Post Office, incompetent as usual, never delivered it to the IRS. By the time we found out and were able to get another check out, it was too late.
If you're sending anything important to the IRS, play it safe and use certified mail.