Character mistake: During the dancing girl scene, as they are singing the praises of Kane, the girl to the immediate right sings out of time with the rest of the dancers during the line "His favorite son." Even though she forgets to sing the line, she can be heard singing it a bit too late. (00:41:00)
Continuity mistake: In the scene with the lovely dancing ladies, Cotten blows cigar smoke just before a cut to him from the opposite angle. From this angle there is no smoke. This takes place just before Wells throws his jacket to Cotten.
Factual error: When we see the Spanish newspaper reporting the death of Kane, the title says "el Sr. Kane se Murio". No self-respecting newspaper would omit the accent on "murió". Also, Kane was supposedly a worldwide household word, so he would never deserve "el Sr.". Finally, the colloquial "se" is out of place in a printed title such as this. The title should simply be "Murió Kane".
Revealing mistake: When the Parrot (or whatever bird it is) cries in a close-up, one can see straight through his eyes onto the sea in the background. Probably a mistake in an optical printer.
Continuity mistake: When the dancing girls start singing to Charles Kane, there is a frontal shot of Kane sitting next to a man wearing a hat. When the shot changes from being in front of them to being behind them, the man is suddenly no longer wearing the hat. When the shot changes back, he is once again wearing the hat.
Continuity mistake: When Kane is talking to Miss Alexander on her sofa the dramatic lighting changes from the left side of his head to the right. During close ups his left site is lit and his right side is in the dark. When the shot is from the front, showing both actors, Kane is lit from the right leaving the left half of his face dark.