Apparently in "Vigil", Paterson Joseph's submarine Commander was sold a boob by the costume department.
He goes from 3 rings (Commander) to 4 rings (Captain) and back to Commander again. Fastest promotion and demotion ever!
And a famous rank slide boob, for all you military uniform buffs out there: "Captain" Jack Harkness in Torchwood is undercover and ostensibly acting as a US Air Force Captain (so an OF-2) on exchange with the Royal Air Force. He wears the rank slides of an RAF Group Captain however (OF-5), so he's promoted himself 3 whole ranks by wearing that rank slide. I hope he's getting the equivalent pension!
Of course the TV producers will have just said... "Meh, Captain, Group Captain, they'll be equivalent I'd have thought."
Oh and his back story and that famous coat is supposed to be from WW2, but that style of greatcoat and those rank slides didn't exist in the 40s. The rank braid was sewn directly onto the sleeve cuffs then. Rank slides on shoulder tabs only came in much, much later.