Continuity mistake: Mark, April and the professor leap from a boat seconds before a bomb goes off aboard. But in the shot of the boat exploding right after they jump, there's no one in the perfectly calm water, and the empty rubber raft is still alongside the burning boat. Somehow, though, the escapees are rowing to safety in that same raft a few shots later.
Revealing mistake: The climactic chase occurs in an amusement park, supposedly in Brighton, England. But several things give away the real filming location, among them an ad for 15c (not 15p) snow cones, two very American-style phone booths, and best of all, a pole proudly sporting both a U.S. flag and a California state flag, visible in the background behind the phone booths.
Revealing mistake: The motorcycle chase is supposed to take place late at night. But in several shots, a cloud-strewn blue daytime sky can be seen, and long shadows on the streets indicate that the scene was actually shot either in the early morning or the late afternoon.
Continuity mistake: The bad-guy bellman knocks Mark out, then zips him face down into a garment bag. He then places the bag, without turning it, onto his luggage cart, and the outline of the body inside (a dummy, judging by how effortlessly he lifts it), is now facing up.
Revealing mistake: Mark hides inside the grand piano - improbable (there's not enough room) unless the piano is a mock-up without any real harp or strings. And in fact, when he jumps out, you can see that there's nothing inside. Somehow, though, the piano still makes sounds when Dieter touches the keys.
Continuity mistake: Merek's bird is sitting on his shoulder facing backward. In the very next shot, as he stands, the bird is suddenly facing forward. When he starts to sit down, the bird is still forward, but in the next shot, a close-up of him sitting, birdie is facing to the rear again.
Continuity mistake: Mark injects April with plain water, telling her to play dead. She shouldn't have to play. Water in the blood stream can create oxygen bubbles, which can be lethal in a matter of minutes. He should have used saline or glucose instead. Those solutions have been de-oxygenated and purified to prevent giving patients the potentially lethal "bends."
Revealing mistake: Waverly opens a file full of papers with a small photo of April's contact clipped inside. When we see a close-up of the photo, we can also see that the page under it is part of a script treatment. Odd thing for a spy agency to keep on file.
Continuity mistake: Highly-trained secret agents are supposed to know their own IDs. When April reports to HQ over her communicator, she identifies herself as "April Dancer, Section One." She works, as do all the U.N.C.L.E. field agents, for Section Two.
Deliberate mistake: The bad guys chase Mark through a movie studio backlot past a London bus with a large red "Out of Service" sign on its side. During the ensuing shoot-out scene, the film is reversed for some reason, and the sign on the bus is suddenly backwards.
Revealing mistake: Mark's stunt double during the fight on the mansion's porch is a very close match for him - except for the fact that he's about 6 inches taller. This causes "Mark's" height in relation to the man he's fighting to fluctuate noticeably (taller than, shorter than, taller than again) between shots.