During a brief conversation with Holmes and Miss Morstan the cream from Watson's interrupted shave moves from his right cheek to his left. (Actually, there is originally shaving cream on both sides; then Watson wipes his right side.)
Tonga is supposedly from The Andaman Islands in India. But he is clearly just a painted man, with blue eyes.
The police inspector played by Terence Rigby is named "Layton", but was supposed to be the more famous Inspector Lestrade. You can clearly see Ian Richardson and the other actors mouthing the name "Lestrade" throughout the production, but for some reason they were all dubbed over with "Layton".
During and after Tonga's fight with Sherlock at the Sholto home, you can see the wires that aid Tonga's leaping abilities.
Holmes is chasing Jonathan Small down the Thames. He passes under Tower Bridge towards the sea but in a subsequent shot he is seen passing the Royal Naval College at Greenwich traveling away from the sea as the college is on the south bank of the Thames.