In the first episode of Season 2 Sydney is shot in the shoulder. Over the next several episodes she wears a bandage on the wound "off-duty" but there is no sign of either wound or bandage when she's wearing disguises that do not completely cover the area.
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!
During the title sequence of Season 1, a moving automobile explodes. The hood flies off just as the tail of the car is sent upward, revealing an empty engine compartment.
During season 4, the shots of downtown Los Angeles are clearly from years past. BP Plaza was renamed Bank of America Plaza in summer 2004, and the Disney Concert Hall is never seen.
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".
In virtually every episode where the characters are shown sneaking around in Eastern European or Asian countries, US-style electrical outlets are visible.
In season 3, Omnifam's office is in Zurich, Switzerland, but the distinctive Munttoren, canal tour boats and the yellow trams of Amsterdam spoil the illusion.
In Season 3, Omnifam'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).
Characters often pronounce the first name of Milo Rambaldi as MY-lo. If he was a medieval Italian, it would probably be pronounced MEE-lo.