Deliberate mistake: I have participated in several amateur karate tournaments over the years and I have yet to see one that is full contact without any protective equipment whatsoever, especially one in which all of the participants are minors. All of the contestants should at a minimum have had mouth guards, gloves and foot pads.
Audio problem: At the Country Club Dinner Dance, the band is playing the "Fascination Waltz", but the conductor's baton is at a faster tempo.
Other mistake: When Daniel, Miyagi, and Alli enter the tournament arena, they are stopped by a referee who says only fighters and coaches are allowed on the floor. They use the excuse of Ali being a translator to get her by. As they move forward you can see children in street clothes, girls with purses, old ladies, and others who are obviously not fighters or coaches.
Continuity mistake: When Mr. Miyagi saves Daniel from the Kobra Kai during the night of the Halloween dance, Mr. Miyagi flips Johnny to the ground and only strikes him in the stomach. The next morning, when Daniel and Mr. Miyagi go to Kreese's dojo, Johnny is seen with a black eye. Very hard to get one since he was never hit in the face.
Continuity mistake: When Daniel and Mr. Miyagi pull up in Mr. Miyagi's car at his house to start Daniel's training, it cuts to an overhead shot. Mr. Miyagi gets out and walks around the front of the car, and passes the front of the car. Yet in the following closeup he is passing the front of the car again. (00:51:00)
Other mistake: When Daniel and his mother have to bump start the car, she shouts to Allie to pop the clutch. There is no tell tale sign of the car reacting to the clutch being dropped, and the sound of the engine starting is preceded by the sound of the starter motor turning as if the key was used.
Audio problem: During the beach fight between Johnny and Daniel right at the time Daniel runs at Johnny and Johnny trips him for the FIRST TIME, you can see some of Johnny's friends holding Ali back as she yells, "Johnny, Stop It." But if you look close, it is easy to tell her lips aren't moving at that point.
Continuity mistake: At the very beginning, when we first see Daniel, he begins to talk with another lad. When Daniel and the other lad stop at the metal stairs to talk to an old lady, the other lad is resting his arm on the handrail and is also at the bottom of the staircase, yet in the following shot he is no longer resting on the handrail and is also a few steps up the stairs from what we saw in the first shot. (00:05:50)
Continuity mistake: During the bathroom scene where Daniel is about to take revenge out on Johnny, watch as Daniel uncoils the hose - it is very long and has several obvious kinks in it. However, as he turns on the water and Johnny runs out, the hose is a perfect straight shot, and MUCH shorter (and water flowing through the hose would not do this).
Continuity mistake: In the scene where Daniel is riding bike home from his mother's work and Johnny and his pals are chasing him on their bikes, they kick Daniel down the hill. As Daniel is rolling down the hill, you can see that it is a stunt double with no cuts on his face and when Daniel re-appears in the shot, his face is bleeding.
Continuity mistake: When Daniel and Mr. Miyagi are in the Kobra Kai Dojo speaking with the Sensai, in one shot the Sensai points at Daniel, who is standing beside Mr. Miyagi. Then in the following shot, when the Sensai is pointing his finger at Mr. Miyagi, Daniel is now standing behind Mr. Miyagi. (00:49:45)
Continuity mistake: When Johnny is walking away from Ali, the shot cuts to Daniel lying down on the sand, and there are two guys behind him, one with brown hair and a red tie around his head and a blonde guy. In the following shot after the line "Where did you find this guy," the two guys are now shoulder to shoulder with the guy in the yellow shirt. (00:13:35)
Continuity mistake: When Daniel and his mother are in their new apartment, Daniel goes over to the sink to fill up a bowl of water. The two items on the top by the sink appear in the first shot, then disappear in a shot from behind Daniel then reappear again in a shot from the front. (00:06:45)