Continuity mistake: In the same part, he is now standing on top of the wall. Zorro takes out his whip and wraps it around the tree limb and swings over to the other part of the wall. When he lands on the wall, he takes off running, letting go of the whip. When the camera returns to him running on the wall, he has a new whip, all rolled up in his hand. (00:08:05)
Continuity mistake: Near the beginning of the movie, after the big fight scene, Zorro rides up the steps on his horse and behind him you can see that the sun is setting, which means it should be early evening. But whenever it shows the boys, the light on them and the landscape makes it appear to be mid-day. (00:09:25)
Continuity mistake: When Alejandro and Diego fight in the cantina, Alejandro knocks over stalks of corn rolled up in a bundle into the doorway. In the next shot, the corn stalks are standing again. (00:35:30)
Continuity mistake: When in the cantina, at the time the corn stalks disappear, Zorro knocks the sword out of Alejandro's hand. The sword seems to fly back behind him. In the next shot, Zorro catches the sword because it is now falling straight from the sky. (00:35:40)
Continuity mistake: When Antonio Banderas is finally proficient at swordsmanship, he knocks Anthony Hopkins' sword out of his hand and catches it. He then laughs and throws it back pointing downwards, but when Anthony Hopkins catches it, it's pointing upwards. (00:41:00)
Continuity mistake: Later in the movie, when Alejandro is now Zorro and in the house confronting Captain Love by drooping in behind him. Watch the tip of the sword as it moves to a different location on the Captain's uniform. (01:32:55)
Continuity mistake: During the Alejandro/Elena fencing sequence, after he kisses her the first time, her left sleeve is down. The shots before and after have it covered. He slices the fabric off in the next shot. Her right sleeve also came down for no reason. (01:38:15)
Continuity mistake: When Alejandro and Captain Love are fighting on the gold lift, Alejandro punches the sword out of Captain Love's hand. He grabs it and stabs it through him. When it shows the top view of the sword stuck in Captain Love, hit the pause button. Look at the sword blade going through Captain Love's stomach.Then look at the other end of the blade going out of his back. They are at different places in him. (02:03:00)
Continuity mistake: Don Rafael's hair keeps changing from being curled to straight when Zorro (Antonio Banderas) first appears before him.
Continuity mistake: After don Rafael show the other dons the map of an independent California, a close-up shot of Alejandro shows him putting his cane under his arm. The next shot, a wider view, shows him doing it again.
Continuity mistake: When Three Finger Jack presents Alejandro and Joaquin to the bumbling Mexican soldiers, he touches a wanted poster and asks permission to read it. He's not touching it any more in the next shot.
Continuity mistake: When Love is stabbed, there's no blood on the sword, but later there is. It couldn't have dripped down, since it's only at the end of the sword.
Continuity mistake: When Love is stabbed, the sword isn't sideways, but in the very next shot, it is.
Continuity mistake: When the dons are all in the mine and looking at the large amount of gold while Rafael is laying out his plan to buy the land with the gold that he mined there, you see Antonia Banderas standing among the dons with Love behind and to the left of him. But, moments later, you see him turn his head and look over his shoulder at Antonio, having not been able to move in that short amount of time, having to push through a few of the dons to get there.
Answer: He recognized that Bernardo was really Diego de la Vega and he realized that the young Zorro he had seen earlier could not have been Don Diego. He then deduced that the new Zorro looked similar to Don Alejandro (and was probably warned by Captain Love that Don Alejandro was hiding someting) and made the decision.