Plot hole: If Noah could be brought back from the dead with Claire's blood why couldn't they bring back Nathan? Was turning Sylar into Nathan really the first thing they could think of?
This One Time at Space Camp... - S4-E18
Plot hole: When Dr. Hughes is racing to save Allison when he thinks he is Carter from the events that happened in "I'll Be Seeing You" Henry says he is racing for a cliff, and that the cabin was just a hologram. But in the episode the Cliff was the hologram disguising the cabin. Henry even commented in that episode that he did not remember a cliff being there.
Plot hole: The Unit discovers a soldier who they think is Betsy, tied to a chair. They note that the room is covered with PIRs or Passive Infrared Sensors. They then take out their night-vision goggles and begin counting the number of "infrared lines" they see. However, PIR sensors don't emit anything; they are passive. (00:23:15)
Plot hole: When Mr. Ballentine is telling Emily about her assignment, he sets the blue folder down near his side of the desk before he gets up. Then we see Emily pull it across the table and quickly peek in it. When Ballentine turns around, the folder is still close to Emily, which shows that she at least tried to look in it, yet Ballentine says nothing about it no longer being in the spot where he set it down. If Emily thought that it was okay for her to look in the folder, she wouldn't have waited for Ballentine to turn around before she touched it.
Plot hole: Owen is unable to give Henry the kiss of life as he has no breath, and yet he is able to talk.
Are You...? - S7-E1
Plot hole: It's out of character and against his code for Dexter to kill Viktor at the airport. Airports have security cameras everywhere (you can see one on the ceiling when he is wheeling Viktor into the unclaimed baggage storage), and using the unclaimed baggage storage as the kill room is equally unwise, because any airport employee could walk in at anytime and catch him red-handed. Dexter was caught murdering Travis Marshall by Deb just hours before, so if anything, he should be even more cautious.
Plot hole: Cleo's photo album contains photos of her and Lewis from past episodes ("Love Potion", "Doctor Danger" and "Hocus Pocus"). It's impossible that their photos could have been taken because they were alone in all of them. (00:03:07)