The Getaway - S4-E12
Corrected entry: After Dexter kills Arthur Mitchell and drops him in the ocean there is a flashback. You can see Dexter, Rita and the kids celebrating Harrison's first birthday in their backyard (there is a 1 on the cake). In season 5 however, we learn that Harrison is only 10 months old, and at the end of season 5, Harrison celebrates his first birthday at the beach in Miami.
The Getaway - S4-E12
Corrected entry: The bathtub Rita is found deceased in is a standalone without a shower door, unlike previous episodes featuring the bathroom with a shower/tub with a sliding door. In a previous episode, Rita refers to the home as a "one-bathroom" house so it's not possible this was intended to be a second bathroom.
Correction: The tub that Rita was found murdered in was in the home that Dexter and Rita purchased together after their marriage. The previous home in which the sliding glass door and "one bedroom house" comment was made was Rita's former home that she lived in with her husband Paul, not the home she purchased with Dexter after they married. They are two different houses.
The Getaway - S4-E12
Corrected entry: If we assume that Dexter took off the oil cap while it was still in the shop for Arthur, it still doesn't explain how Dexter got the jump on him when his car (hours later) broke down. Arthur's not shown to stop, but to merely drive for hours on end until night. Arthur's on a barren strip of road with no other cars or traffic to be seen for miles. It cuts from Arthur looking in his engine to Dexter suddenly walking out from behind the Mustang. The only explanation is that Dexter waited in the car with Arthur, which doesn't make very much sense. All this takes place in the last 15 minutes of the finale.
Correction: Dexter was hiding in the trunk of the car. If you look very closely when Arthur is saying "Where's my f***ing oil cap?", you can see Dexter's leg appear to the left (viewer's right) of the rear passenger side tire, in the open space under the car's body.
The trunk thing makes sense, but they could have made it more clear. I too was wondering how the heck he just appeared seemingly out of nowhere.
Correction: This was a "flash forward." Dexter was imagining what his future would hold now that the threat of Arthur was gone.