I think Thomas.Fredrickson is expressing dismay at Mrs. Obama ending a sentence in a preposition as quoted here:

But we also know that there are new faces coming into this work and we want to welcome you and thank you for the hard work that you're going to put in.
However, ending a sentence in a preposition being a 'grammatical sin' is actually an urban legend propagated by teachers who themselves have succumbed to this fallacy. There is nothing wrong with ending a sentence in a preposition and is perfectly acceptable.