Revealing mistake: From the first "front on" view of Gwen and Owen in the arcade as they are running to catch the alien device, you can see behind Gwen to the right that Owen is being played by a stunt double.
Torchwood (2006)
1 revealing mistake in season 1
Starring: John Barrowman, Eve Myles, Gareth David-Lloyd, Kai Owen
Captain Jack Harkness - S1-E12
Factual error: When Gwen finds Toshiko's hidden message from 1941, she realizes that it was written in blood, and her flashlight reveals the bright red letters on the now-yellowed piece of paper. But after 60+ years, the iron in the blood would oxidize. The writing on the note should be rust-brown, not red. (00:33:40)
Captain Jack Harkness: Captain Jack Harkness, note the stripes.
Captain John Hart: Captain John Hart, note the sarcasm.
Captain Jack Harkness: Hey, I worked my way up through the ranks.
Captain John Hart: I bet the ranks were very grateful.
Trivia: Torchwood is an anagram for Doctor Who, the show of which this series is a spin-off.
Question: How did Jack get the Doctor's severed hand?
Answer: He might have requisitioned or stolen it from Torchwood London, or he might have been ready down below once it'd been severed, searching for "the right Doctor" and not knowing that the newly-regenerated Doctor was the one he was looking for.
Captain Defenestrator