When Holmes is awakened by Mrs. Hudson, a modern-day filter-tipped cigarette butt can be seen in the ashtray beside the bed; filter-tip cigarettes were not widely available in England until the 1950s (and Holmes, as evidenced in the same scene, correctly smokes untipped cigarettes).
(Jeremy Brett was a chain-smoker, and presumably had smoked the filtered cigarette between takes.)
Holmes' decision to keep the Blue Carbuncle for himself results in several plot holes. It is never made clear how he and Watson intend to clear John Horner of the charge of stealing the stone without admitting they recovered it. Furthermore, Holmes keeping the stone also prevents the commissioner from collecting the thousand pounds' reward, which Holmes all but promises him in the course of the investigation.