When Dexter and Lumen are in the restaurant for the second time there is a freighter going by in the background. As Lumen is walking in, the freighter can be seen passing. When she shouts at Dexter and the angle changes; the freighter can be seen approaching again. And as the scene finishes the freighter is seen passing by the window again.
Dexter supposedly goes to the Julia Tuttle Bridge. Actually, the Julia Tuttle Parkway is a sea level causeway with no elevation for people to live under.
Francis claims she always remembers to lift with her back. If she made a habit out of that, she'd be guaranteed to put her back out. You're always supposed to lift with your legs, not your back for exactly that reason.
She was, however, clearly being sarcastic while making this comment.
When Dexter goes to Lumen's hotel to talk to her, someone or something is bunching up the curtain in the corner of the window by the door and Dexter even appears to react to it slightly. But when he enters the room there is nobody there and nothing present that would have caused the curtain to do that.
When Deb and Cira break into the house because they suspect it to be a crime scene, they don't even put on latex gloves and touch one door knob after the other, potentially destroying fingerprint evidence and contaminating DNA.
Dexter has not told Lumen his last name, and probably not even his true first name, but when he hands her the plane ticket, he says he "used up some miles". If that were actually true, it would have created a hard data trail and be potentially even worse than telling Lumen his actual name. Either that or he collected those miles under a well-crafted fake identity. On the other hand, he would even have needed one of those to buy a plane ticket for cash without implicating himself.