When John Druitt appears with his arm inside Nikola Tesla, the camera zooms in on his right hand clearly in Tesla, however in the next scene after transporting to safety, his left hand is covered in blood and his right one clean.
This episode's flashback to the Five's time at Oxford in the 1880s is wrong on two counts concerning Nikola Tesla: First of all, he had a Slavic accent at the time (which he kept until his death in 1943), not an American one. Secondly, he never studied or taught at either Cambridge, Oxford, or Eton. In fact, he didn't even set foot on English soil until the late 1930s.
John refers to Nikola Tesla as Nicholas.