Walter asks his former neighbor if she "still" has keys for Skyler's house. She does, however, Skyler had changed the locks earlier in the season.
When Walt and Jesse exit the lab after rigging it to blow, there is a plume of smoke billowing from the lab's entrance in the laundry. The timer Walt set to spark the blaze didn't engage until after Jesse pulled the fire alarm.
When Gus arrives as the nursing home, it shows him parking in front of the building (next to the handicapped spaces), but a few minutes later, he gets out of the car and walks across the parking lot (and through the landscaped circle) to get the front door. Additionally, the landscaped circle is differently landscaped from the overhead shot to the this shot.
The agent flips the top page of his notebook twice during the meeting with Tio Salamanca and Schrader.
The same hospital staff member who walks off the elevator with Walt walks back onto the elevator moments later.
During the explosion at the nursing home that claims the lives of Gus, Tio and Tyrus, only the explosion is seen, no fire at all. However during the TV broadcast seen at Walter house says the fireball completely consumed the room. The fireball would likely have been seen when the door was knocked off the hinges.
The striker button on Hector (Tio) Salamanca's bell is bent in all other episodes except for this one. In this one the striker is straight, but this may due to Walter modifying the whole setup in order to set the explosives with the striker as a trigger. To avoid a failure, he might have wanted to assure the proper functioning of the striker.
The cotton plug inserted into Bryan Cranston's right nostril used to make his nose appear swollen under the bandage (following the car wreck with Hank) is visible in a shot when he is on the roof of the parking deck of the hospital in the last scene when he is talking on the phone to Anna Gunn.