Revealing mistakes: During the title sequence, a moving automobile explodes. The hood flies off just as the tail of the car is sent upward, revealing an empty engine compartment.
Errors in geography: One episode has brief scene in Helsinki, Finland. The scene involves high skyscrapers, but no such buildings exist in Helsinki.
Factual errors: In the first season, when Sydney and Dixon go to Sao Paulo, Brazil, the locals correctly speak Portuguese in this episode. However, the subtitles say that they are "speaking Italian".
Errors in geography: In season 3, when transitioning to Sloan's office, the images shown are supposed to be of Zurich, Switzerland, but the distinctive Munttoren, canal tour boats and the yellow trams of Amsterdam spoil the illusion.
Errors in geography: In many of the episodes when Sydney visits other "countries", one can identify that filming took place in the US because of American telephones, traffic lights, types of signage, types of cars, architecture, door frames and door knobs in the background. Those are subtle for the most part but people living in the countries depicted feel that "something isn't quite right".
Revealing mistakes: In season two, Sloane's wife Emily is supposed to have had her ring finger cut off as part of the plan to fake her death and disappearance; indeed, we see that it is missing in "Phase One". However, her ring finger is visible and "intact" in several scenes in later episodes, including one in "Truth Takes Time".
Factual errors: When Sydney is seen on a speedboat in Greece during the first season, she asks a man on another boat for help. The caption reads (DVD) "Speaking Russian", and actually she is speaking Swedish. What she says is more or less "Ursäkta mig. Min båt... Kan ni hjälpa mig? jag har ingen bensin kvar," and then "Tack så mycket.". = (Excuse me. My boat... Can you help me? I'm out of gas," and then "Thank you so much."
Continuity: There is an episode in the first season where Sydney hides from a henchman by climbing onto a large pipe overhead. As the pipe begins to heat up and scald her hands, the henchman unlocks the deadbolt to an office door, leaving the keys visibly dangling as he enters. Just as the door closes, Sydney drops down in front of the door, but the keys are missing.
Errors in geography: In virtually every episode where the characters are shown sneaking around in Eastern European or Asian countries, US-style electrical outlets are visible.
Errors in geography: In the Episode where Sydney and Dixon go to Guadalajara the first take shows and auditorium front face with the name "Auditorio de Tijuana".
Miscellaneous: Sydney Bristow language abilities: The Czech and Hungarian languages sounded absolutely ridiculous on the show, not to mention all the pronunciation mistakes.
Miscellaneous: Despite being a prisoner of the state, Irina wears peach lipstick and black eyeliner in nearly every episode.
Errors in geography: In season four, when Sydney goes to São Paulo, Brazil, they show one beach image. There are no beaches in São Paulo.
Errors in geography: In Season 3, Sloane's office supposed to be in Zürich, Switzerland. The exterior shot right after the "Zürich" insert is a famous building in Budapest, Hungary, on the Astoria Square. The shot is borrowed from another film, Spy Game (2001), where Budapest played Berlin. (In Season 5, when the plot moves to Hungary, another shot was used, which is correctly in Hungary - Bem rakpart, the coast of Danube in Budapest).
Factual errors: Many episodes feature a sequence in which a long-distance sniper uses a laser sight to hone in on a target, but a real sniper would never even consider using one. First of all, the laser gives away the presence of the sniper which, although used successfully in the show, completely defeats the purpose of a hidden sniper. Second, and more importantly, a laser sight is completely worthless to a sniper because there are so many factors that affect the trajectory of the bullet on long-distance shots - wind, altitude drop, and the Coriolis effect on very long shots to name a few - and have no affect whatsoever on the laser that the bullet would wind up being at least several feet, if not several yards, away from target the laser pinpoints!