In educational circles, National Geographic publications usually rate pretty highly as a reference source. On the other hand, its cable television channel, the National Geographic Channel, can be a bit hit and miss.
By the time she performed at President Kennedy's birthday gala, Marilyn Monroe was on a downward spiral. She died three months later. It is well known and thoroughly documented that she died at her home in Brentwood, Los Angeles.
The documentary says that she was found dead in a rented New York apartment not long after the gala. Errors like this are completely avoidable in this era of information overload, where it is simple to check and double check your sources. There's something to be said for diligent and thorough research.