When Jack falls down the building, clearly there is nothing down there. But when he hits the ground, he falls on a stone bench.
After Jack and John fight in the bar, the blood on John's lip alternates between a thin, neat line and a smear.
During the standoff with Captain John in the Hub, Owen suddenly goes from holding his gun with one hand to holding it with two.
The slide on Gwen's gun is locked back while the team is advancing on Captain John Hart in the bar. Thus, Gwen's gun is clearly empty.
"Hologram Woman" states that the bomb attached to John's chest will blow up in 10 minutes. Ianto doesn't take out his stopwatch and begin timing until she says this, which is 10 to 15 seconds AFTER the bomb has attached itself to John. So at the end when Ianto is counting down, "4...3...2...1", the bomb SHOULD have already exploded.
After Gwen punches Captain John Hart, Ianto is heard to say "Remind me never to get on your bad side." but his lips aren't moving.
On Tosh's screen when she is analyzing the poison in Gwen's system, the neurotoxin synanceia - derived from the venomous stonefish - is misspelled as "synanceja"