Bet I can guess a few.
- That Deb agreed to burn down the church, and went to buy the gas.
- That he not only lost the blood slide but he left a big old piece of plastic between the guys toes and Masuka didn't spot it (actually that's 3 things).
- That the bad guy left his laptop, powered on with the browser open, and Dexter was simply able to pull up the airline ticket purchase on it.
- Deb just caught him killing someone, is giving him suspicious looks and questioning him about the ice truck killer and the same day he decides to kill someone.
- That airport customs let him go with his needles without checking him out. Not to mention he had a tiny little bag that had needles, a plastic apron and rubber gloves in it but no clothes or anything else an international traveler would have.
- That he managed to get the body out of the airport.
Actually the entire airport part was ridiculous. Dexter has had gaping plot holes pretty much every episode for years now but I still like the show.