At ~1:09:20 Johnny has slipped from the marching band to enter the castle. Before going up the flight of stairs he tosses his bagpipe back out the door he just came in. When the scene switches back outside with the security team coming after him, no bagpipes can be seen on the steps or ground.
The Montblanc Blaster is stated to explode after 20 seconds, but it takes as much as 40 seconds before it explodes.
Around 00:54:01, Ophelia is putting a gun on Johnny's temple. On the next shot, she's putting it much higher.
Bough gives Johnny a list of all the guests that used the wi-fi that day. However the hack came from the yacht moored nearby, the yacht was able to access the wi-fi but wasn't recorded while all the guests were.
In G-12 summit, Pakistan is shown as a member nation whereas Pakistan is not one of G-12 Countries.
There is no single point of failure for the Internet. Turning off a single junction box would then cause the Internet to re-route traffic.
It would be impossible for a person to put on a medieval suit of armour by himself. Straps and joints interconnect multiple separate pieces, which often cannot be reached by the wearer. It would take at least one other person to help don a suit of armour, which is why medieval knights had squires.
A suit of armour, whether genuine or a reproduction, would not stop a modern bullet. After the development of the flint lock musket in the 17th century, the use of armour declined.
Volta refers to the traffic control hack as a Denied Service Attack, this should be a Denial of Service Attack. Volta's phone actually displays "DDoS" indicating a Distributed Denial of Service attack.
English refers to the senior officer on the submarine as 'Commander' but she wears 4 stripes on her sleeve denoting her as a Captain and at around 28 minutes in to the film, Bough said that he was married to a Captain of a nuclear submarine. However, given his general incompetence this isn't out of character.
In the sandwich shop when English is swinging the two baguettes against the staff person, a real baguette would have broken into many pieces quickly. The baguettes in the movie were obviously rubber props.
When the headmaster first comes looking for Johnny English, you can clearly see that he is not standing in camouflage against the wall, as the next shot reveals him doing.
Johnny English is in the turret of the submarine and addresses the vessel's captain as Commander (a rank below Captain in the British Royal Navy). Later she is seen wearing the insignia of a captain on the sleeves of her jacket.
Bough's wife refers to him as Jeremy when peering out of the submarine's hatch. However, it is established in the first Johnny English film that his name is actually Angus.
The Prime Minister says that English is accusing Volta of "high treason". But Volta is an American and couldn't be treasonous against Britain. Furthermore, as petty treason is no longer a distinct offence, what was previously called "high treason" is now simply referred to as "treason".
When the Prime Minister lists the actions in English's file, she mentions him commandeering an open top bus before assaulting people in a sandwich shop, but the events in the sandwich shop happened before those on the bus.
When Johnny English steps out of his Triumph Dolomite, he walks away from the car without locking it. Later, as he is planning to drive off in it again, he very visibly and audibly unlocks the driver's side door with a key.