When Castle is in the observation room with Cosmo in his arms, watching Beckett and Esposito question Jimmy the Wolf, Nathan Fillion ad-libbed "That's a grown-up lying. Get used to that".
The church into which the victim stumbles at the beginning of the episode is the same church where Ryan and Jenny get married in "Til Death Do Us Part (2012)".
Kristen Ariza, who plays the Child Services rep, is a friend of Seamus Dever's. Years earlier, they appeared together in a play; during a performance one night, in an amusing coincidence, Stana Katic happened to be in the audience.
It is revealed that "Cosmo", the alias Castle gives Baby Doe, is what he was going to call Alexis if she had been a boy.
The baby high fives Castle.