I reckon it depends on what was in the memo. It is a written message that is timestamped and therefore a legal document (Ask Hillary Clinton about emails). So, if your memo was a warning, it is considered a written warning and depending on severity of the infraction, yes, the next time could mean your termination.
SOME PEOPLE TRY TO TURN BACK THEIR ODOMETERS. NOT ME. I WANT PEOPLE TO KNOW WHY I LOOK THIS WAY. I'VE TRAVELED A LONG WAY, AND SOME OF THE ROADS WEREN'T PAVED. ~ Will Rogers