Castle consciously realizes for the first time that the pitch of his voice rises when he finds an interesting or shocking twist in a case.
During his cross-examination of Castle, Caleb Brown mentions that he witnessed the "murder" of Eva Whitfield on February 2, 2015, which is the real air date of "I, Witness (2015)".
When Beckett is helping Castle with his trial prep, he makes a passing mention of Superman. Stana Katic voices Lois Lane in Superman: Unbound (2013).