When DCI Hopkins is shown the grenade Lestrade had at the cemetery it has a red pull ring. When it was at the cemetery, the grenade had a silver pull ring.
Carrier pigeons, or more correctly homing pigeons, only fly home. You cannot attach a return message to the leg of one after it arrives home and then let it go expecting it to take the message back - yet this is what Holmes says he saw.
The plastic powder used by 3D printers contains no dye, so if it was dissolved, the liquid would be clear. It appears white as a powder for the same reason sugar & salt do (because of refraction by the rough edges), and they also dissolve clear.
Sherlock Holmes would never call a Homing Pigeon a Carrier Pigeon. The modern Homing Pigeon is a distinct breed of pigeon separate from the modern Carrier which has been bred into a show bird only and has a completely different appearance.
Sherlock says the firing pin (nail) strikes the bullet. The bullet is the projectile; a firing pin strikes a primer, or the rim on a .22 caliber round. Saying the firing pin strikes the cartridge is 'okay', but not the bullet. Sherlock is precise!
At the end of the episode, after seeing Lestrade in the news, a weatherperson describes the weather as "82 degrees". Fahrenheit has not been used primarily in the UK for decades, and if mentioned at all would be stated after Celsius and specified as "Fahrenheit".
A huge American-style casket is being buried, instead of a British-style coffin.
They arrive at New Scotland Yard, by the Thames. But New Scotland Yard is on Victoria Street - nowhere near the Thames (which they had anyway just driven over and away from) - and the revolving sign is actually down a little side street by St James Park underground station.
When Holmes and Watson are sitting in DCI Hopkins' office at 'New Scotland Yard' the view outside the window is of the River Thames in sight of Tower Bridge. New Scotland Yard is located at St James's Park and is several miles from Tower Bridge.
The investigators would have noticed the strong smell of the acetone in the refrigerator. Even if they didn't recognize what it was, they'd know no food would smell like that , fresh or rotten. The contents would have been thoroughly examined.