Bishop is sent to England, when only two episodes before in the episode 'Twenty Klicks' Gibbs says that only Senior Field agents had clearance to go abroad.
In the bar scenes for Dr Mallard's leaving party, the union flag is hung backwards. Going clockwise, the leading diagonal white stripe should be broader than the trailing white stripe.
During the first flashback in the train station with young Ducky, the tannoy announcement says that the next departing service is for K Street. This wouldn't occur in the UK, as there are no streets with a single letter, much less stations with such names.
When Ducky flashes back to his youth, he is in a train station. This is supposed to be from long ago, however you can clearly make out two modern dome security cameras and some emergency lights on the walls.
When "Ducky" and Bishop are on Angus Clarke's computer in London they say that Angus sent the e-mail to Colpepper but the computer says "From" Colpepper..."To" Angus Clarke.
McGee and Ducky mention an address in Beaconsfield, England with a street number 6321. Whilst it's reasonable that McGee mispronounces the town as "Beeconsfield" instead of the correct "Beckonsfield", no house anywhere in Britain would have the number 6321, because block-based numbering is not used.
Angus' hotel is shown to be rather close to the Thames and Westminster. However, when the team are requesting backup for Ducky, Tony asks for police forces in Finchley. This would still be the Metropolitan Police, but Finchley is also located about 10 miles north of Westminster.
When Ducky is waiting at the station in flashback and Maggie walks in, there is an announcement for the next departure over the tannoy as they converse. The announcer states that the next departure from platform 3 calls at "Canterbury East, Ashford, Folkestone Central, Dover Priory, Dover Western Docks". Whilst these are all stations in Kent accessible from London it would not have been (and still is not) possible to take a single train from London to Dover via this route. Although there was once a line between Canterbury and Folkestone it actually ran from Canterbury West via South Canterbury on the Elham Valley Line, but this was pulled up in the mid 1950s and trains did not run direct from London to Dover via this line. Additionally trains from London via Ashford to Dover have always split at Ashford, usually into 'front 4 to Folkestone, rear 4 to Canterbury West' (not Canterbury East) - Ashford actually being the first of the stations listed on the announcement and being the last on the shared portion of the journey. Depending on which London station Ducky is supposed to be in (and sadly even small UK stations look nothing like the one portrayed in this scene, never mind a major station which would have been the starting point of Ducky's journey!) the train may also have called at additional London stations - for example the start of the line via Ashford would have been Charing Cross, the next station being Waterloo East, followed by London Bridge after which the train would call at Sevenoaks, Tonbridge, Paddock Wood and Ashford then Canterbury West OR Folkestone Central and finally to Dover. Alternatively, if the train was going via Canterbury East then the London station would have to be Victoria and the train would be calling at Rochester, Chatham, Gillingham, Sittingborne, Faversham, Canterbury East, Dover Priory. And finally, although in its later years the station at Dover docks was indeed called Dover Western Docks, it was only named this in 1979 and prior to this was called Dover Marine. If Ducky was travelling via Folkestone there is also just as much chance that Ducky would have been travelling to Folkestone Harbour on the boat train as continuing to Dover.