The Crab with the Golden Claws: Part 1 - S1-E1
Continuity mistake: In Thomson and Thompson's office, one of the detectives sets some files down on the desk. Just before the files are set down, to the left of the files is a green mat with a piece of paper near the end of it. In the next shot, after the files have been set down and one says "Now where on Earth could that file have got to?", the piece of paper on the green mat has vanished. (00:03:25)