Nanny McDead - S1-E2
Revealing mistake: In the very first close-up of the body at the morgue, the 'body' blinks. (00:11:15)
Revealing mistake: When Kate gets the info that the killer is the same one who killed her mother, the back of her shirt is shown twice, revealing each time that it is fixed with needles to fit perfectly.
Revealing mistake: When Castle arrives at the murder scene and views the victim's body, the "corpse's" eyelids are twitching.
Revealing mistake: After Castle and Beckett are arrested for interfering in Chief Brady's investigation, a deputy tells them that Brady wants to speak to them; Beckett stands up to leave the holding cell, but is held back by the pair of handcuffs chaining her to the bench. The cuff is not actually on Stana Katic's wrist; she's just holding it in her hand.
I, Witness - S7-E13
Revealing mistake: When Castle answers his phone at the start of the episode, his phone is on the lock screen instead of in call mode.
Revealing mistake: When Detective Ryan calls Captain Gates after apprehending Jerry Tyson at Michael Boudreaux's apartment, Ryan's phone is not in call mode.
Revealing mistake: At the beginning when the murder victim falls through the ceiling of the elevator, there is a bag of blood that show right behind his head.
Answer: The networks are now having winter breaks. A mid season end of new episodes, when they come back after the new year, it picks up as if several months have gone by. Beckett suggested Valentine's Day as a "chick flick" to watch as she is moved in. Also some episodes are not scheduled to air in order.