In the bed at the beginning Castle says to Beckett "you were right, i had no idea" it is referring to the first episode of the first season (01x01), when Castle tells Beckett it's a shame that they couldn't be together on a date, Beckett answers Castle whispering at his ear "you have no idea".
Stana Katic (Kate Beckett) and Jack Coleman (Senator Bracken) have both appeared in the TV show 'Heroes'.