When DCI Barnaby and DS Scott are interviewing Carter, the security guard, in his home, the wall clock remains stopped at 9:55 throughout the scene, even though the pendulum is swinging steadily.
Power of attorney requires witnesses, generally an attorney. The form that Dexter Lockwood had did not have any signatures or stamps other than Amelia Plummer's.
The jars are stacked orderly on pallets without being boxed, or at least wrapped in plastic. Jars of relish would be packed in a case before being stacked and the whole pallet wrapped in plastic shrink wrap to keep it together.
Also, the way the pallets were stacked, there should've been dozens of shattered jars.
The jar sterilizer uses super-heated steam to sterilize them. The temperature of the steam is stated at 200°C. However, the boiling point of water is 100°C, and boiling is often seen as enough to sterilize even some degree of contaminated water. (i.e. boil advisories)
When Barnaby and Scott go to visit Ralph in the blind, he incorrectly pronounces his name as 'Ralph'. A few seconds later he pronounces it as 'Rafe' as everyone else does.