Over the course of the episode, Jack's body, torn apart by the bomb at the end of the last episode, recomposes itself gradually, and regrows its skin, which is at first missing (burnt off by the explosion?). A scene early on in the episode however shows Agent Johnson retrieving Jack's severed arm, with the skin on it perfectly intact. Later on, when we see Jack's body in the morgue recomposed but without any skin on it, that same arm, like the rest of the body, no longer has skin on it.
Jack is first held in a cell about 10' square chained down to a table when it is filled with concrete. Later in the episode, the concrete block that is ripped out of the wall and dropped over the cliff is barely big enough to hold an upright man.
The result of the explosion is shown as a crater more or less centrally situated in Roald Dahl Plass. However, Jack was standing near the bottom of the invisible lift which is near the water tower, to the side of the Plass, when the bomb exploded.
While trying to rescue Jack from the prison, Gwen is involved in a firefight with some guards. Using a cell door for cover, she fires a number of shots. The point of view switches several times during the scene. When shown from the front, she is using her right hand and facing her targets. When seen from the back and you can see who she is shooting at, she is turned around with her back to the door and is firing by using her left arm swept behind her.
As Frobisher, Lois, and Bridget, seen from behind, start up the stairs together, two other people come down past them. In the reverse shot, when they are less than halfway up, the other people have vanished.
The atmosphere pumped into the sealed chamber built to the 456's instructions contains a number of poisonous gasses, including "12% fluorine". Fluorine, (atomic number 8) is the only gas more reactive than oxygen; flammable substances exposed to pure fluorine at room temperature are likely to burst into flame. It is quite likely that the mixture described might well explode as soon as it was mixed.
When Jack's cell is filled with cement he is lying on a gurney that spans almost the entire length of the room. Therefore, it is possible that when Ianto drops the cell off the cliff that the gurney, depending on the metal it is made from, could have created a natural fault line within the cement that would allow it to crack open around Jack's body, leaving him one piece.
When Ianto and Rhiannon are at the park and all the children stop, there is one child on the roundabout that continues to move when they're all supposed to be completely still.
There was no reason for Frobisher to take Lois to the construction site. He had Bridget with him for assistance, the Home Office was struggling with many phone calls Lois was meant to deal with, and he wanted to keep everything secret; he actually ordered her to stay outside.