When Acheson visits Jones' office, the glass in the door says the office is of the Director of the Metropolitan Police. Jones is elsewhere referred to as Chief Inspector. There is no such position or rank as Director in the Met.
When Frank jumps from the balcony into the fruit market, escaping his chasers, you see him going down with his feet in front, landing on his heels. He would then come to rest on his behind. Instead, in the continuity cut, he jumps forward, pushing the policeman into the canal.
In the train sequences early on, the trains shown are electric high speed trains. However the sound effects heard during the aerial shots of the trains heading from Paris to Venice are that of diesel power cars of British InterCity 125 trains. Similarly, in the train interior scenes, no overhead power masts can be seen through the windows. The lines from Paris to Venice are all overhead electric powered.
Shaw's goons clearly have little experience or training with using firearms, as they all attempt to fire their handguns at arm's length without bracing the firing hand, and without even sighting down the length of the barrel.
When Elise drops Frank off at the airport dock in the early morning, she is wearing a beige sweater and black slacks. As she boats away from Frank and across the bay, she has changed clothes and is wearing a gray dress, heels, and gloves, and it is much later in the day.
In most of his scenes, Acheson is referred to as "Commander", a senior rank in the British police force. By the end of the film, he is referred to as "Inspector" by Jones, who as a Chief Inspector outranks an Inspector but is junior to a Commander. Yet there is no scene showing Acheson being demoted.
When Elise is towing Frank from the boat in the water, you can clearly see him with unhand-cuffed hands pulling himself in towards the boat. In the following scene Elise is then picking the locks on the handcuffs.
When Frank is pulled into the water while hand cuffed to the boat, after kicking off his attacker and being dragged into the distance leaving the gangster behind.
In the next shot, where Elise's boat is in the background, a camera operator/assistant can clearly been seen from emerging from the 'passenger' compartment of the boat.
When they are on the train going to Venice through a nice hilly landscape, the speaker says "Venice in 10 minutes". There are no hilly, agricultural landscapes near Venice, only a semi-industrial plain for tens of kilometers.
When Frank is walking on the roofs of Venice, he sees Elise at the other side of the Grand Canal, near the market, but a few moments later he is on that same side of the Grand Canal and jumps from the market building upon a market stall.
When Frank flees from the Danieli, he runs across a few rooftops and then jumps onto the roof of a stall in the market. The market is actually quite some distance from the Danieli and on the opposite side of the Grand Canal.
When Frank goes out on the balcony of the Hotel Danieli you can see the Rialto Bridge, which is not near or visible from the Hotel Danieli. This is because the Palazzo Pisani-Moretta, from which the Rialto Bridge is visible, was used as a "stand-in" for the real Hotel Danieli.
The night Frank sleeps on the sofa he dreams about walking into Elise's room and kissing her.
The dream shows the interior of the room exactly as it is in real life.
However Frank has never entered the room to know what it looks like in his dream, so he would have imagined things differently to how they were in the room and not exactly as it was in real life, something he had never seen.
Shaw refers to killing his wife's lover as a "crime of passion" and explains that "under [England's] quaint legal system" he got away scot-free. The courts of England & Wales do not recognize "crimes of passion" (though many other European court systems do), although it may have been accepted as mitigating circumstances and have possibly contributed to a reduced sentence due to diminished responsibility. Shaw would still have been looking at a lengthy jail sentence for manslaughter, at best.
The goof items below may give away important plot points.
One of Reginald Shaw's bodyguards who was slain by sniper-fire is seen slumped face down on one of the sofas. In the subsequent scene, all the slain bodyguards are sprawled on the floor along with Shaw.
When Elise leaves Frank at the dock, the boat she's driving has a "TAXI" sign on the windshield. When she arrives at the police secret base in the very next scene the sign is still there, but will disappear when she docks.