Frank's mention of being a Marine with service in Vietnam. During Magnum, P.I.'s run, Tom Selleck often wore a cap with the legend "VMO-2", which was a Marine Forward Air Control squadron out of Da Nang.
Jennifer Esposito has coeliac disease and due to the condition, could no longer keep up with the grueling filming schedule required for her character. While the show writers were willing to work around it, CBS was not and told the show to write her out, a decision she was not happy with.
"Quid Pro Quo" was shot shortly after Hurricane Sandy hit New York City. The episode has Danny pay a visit at one point to a detective friend of his in the Rockaways, the part of the city hardest hit by Sandy. The shots of course being done on location in the Rockaways.
You Might Remember Me from...: Tom Selleck's role as Police Commissioner Frank Reagan is his first major role in quite a while, apart from a string of TV movies based on the Jesse Stone book series. He mentioned in an interview on The Talk that he took the job partly because the script made him cry.