As the occupants of the castle crowd the ramparts to repel Van Zan's approaching column, the men all have their weapons up, ready to fire. If you look over Creedy's shoulder, the man beside him has neglected to flip up the foresight of his SLR, making it impossible to aim the rifle properly.
The type of helicopter carrying the main characters appears to vary from shot to shot during the journey from Northumberland to London, sometimes appearing to be an Agusta A109 with a conventional tail rotor, but in other shots it appears to be an Aerospatiale Dauphine with an enclosed "fenestron" tail rotor.
The harpoon gun that Van Zan uses to bring down the dragon attacking the castle has a bent targeting reticle when we see it from the point-of-view shot as it is fired. However, when we see the gun from the reverse angle, as Van Zan mounts it, the same targeting reticle is a perfect circle.
The American convoy leaves Northumberland (North East England) and travels south until the road is too blocked for them to continue. The battered road signs says "A5 - London 66 miles". The A5 runs from London north west to Holyhead in Wales. Northumberland is 300 miles north of London.
In the opening scenes, the small boy appears to be walking in London around the new Underground building site, but clearly in the background two red and white chimneys are visible. These chimneys are located in Ringsend, Dublin (where this scene was filmed), and not London.
Van Zan and Alex tell Quinn that there is only one male dragon and, like fish, the females lay their eggs and the male makes a "pass" to fertilize them. Yet when Quinn discovers the egg inside the downed dragon, there is an embryo visible, which should only happen AFTER fertilization.
Van Zan tells Quinn the explosive crossbow bolts are magnesium-tipped C4. Assuming the magnesium is being lit from impact or from the dragon's breath, the C4 would not detonate; C4 detonates only with combined heat and pressure usually only obtained from detonating a blasting cap embedded in the C4. Unless the magnesium tip was purposed as the ignition source of a blasting cap that wasn't mentioned in the arrow head the C4 would not have detonated.