What’s wrong with this picture?

I received the above envelope from a Boston area eBay seller containing stamps I purchased for my collection. 

At first blush, the envelope looked innocuous: three commemorative stamps, 4¢, 20¢ and 25¢ denominations adding up to 49¢, the current rate for First Class Mail for 1-ounce letters.

I did not notice anything unusual until I took scissors to the envelope to clip off the stamps. Only then did I notice that the 25¢ denomination stamp was not a U.S. stamp; rather it was a United Nations stamp (Scott #284 from 1977)!

UN stamps (or any foreign country’s stamps) cannot be used for postage for mail originating in the U.S., yet this one got through the USPS system with a Boston, Mass. postmark.

Believe it or not!