When Sir Eustace is killed and falls on the rug, the bloodstain prop actually *tilts up* indicating that it is just a plastic red squiggle.
After about 25 minutes, when Holmes climbs up the mantelpiece to investigate the cord with which one could ring for a servant, there is a low voltage cable running on top of the mantelpiece. This cable has a modern day synthetic insulation, and is attached with plastic clips.
The carriage in which Holmes and Watson ride after the initial visit to the Grange is painted in malachite green, and has the initials ST on the mirror. The Southern Railway was only formed in 1923.