When Jack and Gwen first find Rhys dead on the floor, Rhys already has a bloody hand print on the back of his left shoulder, possibly from Gwen touching him in an earlier take of the scene.
Jack goes from lying cold and dead on a slab, wearing a hospital gown, to nattily dressed and armed without passing through the main work space of the Hub, as evidenced by the surprise of the staff at seeing him emerge from the morgue with Gwen.
When Ianto sees Lisa in the vaults after he closes his eyes and the lights flash and she disappears....she doesn't quite disappear; in fact, her arm and hip are visible in the far back right.
When Owen turns to leave the Hub after being relieved of duty, a few script pages can be seen in the right back pocket of Burn Gorman's jeans.
When Jack and Gwen discuss Bilis, he refers to Bilis as "your dance hall buddy" but Jack also met him while back in 1941. While not incorrect, it's an odd choice for Jack to not say "our" dance hall buddy, since Jack routinely takes the "we, not you" point of view regarding the team's activities.