Continuity mistake: In the final fight between Daniel and Johnny, there is a time when Johnny's head band falls off (after Johnny scores his second point). Right after it falls off, if you look quickly you can see it back on, then it goes back off. (02:00:50)
Continuity mistake: Before the dance in Mr. Miyagi's work area you can see the red shower curtain with the polka-dots that Danny wears to the dance hanging on the wall. After the dance, when Danny is in the work area after getting saved from the Skeletons, you can still see the shower curtain on the back wall.
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.
Other mistake: The end tournament is supposed to be only the very best trained fighters in brown and black belts. Yet the majority of opponents against the Cobras are easily defeated, with weak tactics, unlike Daniel-san and the Asian guy, who actually use fight techniques.