The "voice" of Rumplestiltskin in Emma's head makes a comment about the incomprehensibility of Merida's thick Scottish accent. Robert Carlyle is Scottish himself and has his own heavy Glasgow accent.
Henry calls the nurse in the hospital where Zelena is being held "Nurse Ratched," the name of the nurse in "One Flew over the Cuckoo's Nest" by Ken Kesey, and the patient mopping the floor is similar to the character of "Chief" Bromden.
When Sneezy is acting as sheriff while most of the cast was in Camelot, he has the same red leather jacket and same beanie/hat as Emma.
Sir Percival's coat of arms emblazoned on his tunic is one of the emblems of the band "The Grateful Dead."