December 19 - S3-E10
Continuity mistake: After Johnny picks up the small spear in the dojo, the first spear (next to the one picked up) changes angles (tilting positions) every time it's shown during the scene.
Revealing mistake: When Johnny shows up to Cobra Kai dojo and all the kids are there. In the video playback they show on the phone you can see Miguel didn't land the kick, and the kid just fell down. (00:28:18)
Continuity mistake: Miguel kicks and breaks a staircase railing during the school brawl. For a second it cuts to Robby. When it cuts back to Miguel, the railing is fixed.
December 19 - S3-E10
Revealing mistake: While Kreese is fighting Johnny, the stunt double is clear in several shots.
oswal13
Revealing mistake: When Sam and Tory confront each other at school, the same extra (a girl wearing glasses) appears in two different places. She is behind Sam during the confrontation but Robby also runs past her in another shot.
Continuity mistake: Johnny's car gets hit, and he comes up and starts banging on the windows. When the girls drive off, the rear window is down, when it wasn't prior.
manthabeat
Other mistake: Hawk gets a mohawk, but his hair was too short to style it like that.
oswal13Continuity mistake: When Johnny leaves his home to go to Daniel's place it's night time but when he gets there suddenly it's daytime.
oswal13
December 19 - S3-E10
Continuity mistake: Tory's and the guy's hair behind her looks more disheveled at the start of the house brawl. When she shouts "No Mercy!", their hair looks better. The intro must have been filmed after the brawl.
The Good, The Bad, and the Badass - S3-E8
Continuity mistake: In the beginning of the episode when Daniel and Johnny try to pick up Robby from juvie, the background shows that the weather changes in an instant. When they're arguing, it's raining. When Robby refuses their help, it's sunny.
Audio problem: The man Larusso neglects to sell his dealership to thinks he just got the deal before being punked. He turns around to talk to his assistant but the audio here is off enough to be noticeable. His mouth doesn't match briefly.
manthabeat 
December 19 - S3-E10
Continuity mistake: When Ali dishes about Johnny and Daniel's high school rivalry, she holds her wine glass in one shot but not in the next. It's noticeable even when the camera is not on her.
December 19 - S3-E10
Continuity mistake: When Ali dishes about Johnny and Daniel's high school rivalry, she is sometimes moving just one hand in one shot then it's both hands in the next shot.
Continuity mistake: During the school brawl, Miguel tries to stop Tory from further hurting Sam but Tory kicks him aside. Tory's jacket comes off her left shoulder. It then cuts to Miguel and when it cuts back to Tory, her jacket has come off both shoulders.
The Good, The Bad, and the Badass - S3-E8
Continuity mistake: When Sam and Miguel are "fight flirting", Robby has his right arm around Sam and his left hand is touching her. In the next shot, his hand is no longer touching her.
Obstáculos - S3-E7
Continuity mistake: When Daniel and Sam are fishing, the way that Sam holds her fishing pole changes between close-up and wide shot.
Obstáculos - S3-E7
Continuity mistake: During the dream fight scene, Tory kicks Sam onto a platform. In the wide shot, Sam turns to the left after getting kicked. In the closeup, she isn't.
Continuity mistake: Miguel and Hawk are in the same class when Tory hijacks the announcement. When she meets Sam in the hallway, Hawk is now in the crowd behind Sam, but Miguel is just leaving the classroom and running through the halls.
Continuity mistake: During the school brawl, a teacher tries to break it up only to get body slammed. Hawk is flinching at this in the close-up. In the wide shot, he is kicking someone.
December 19 - S3-E10
Continuity mistake: At one point during the first meeting of Vanessa and Ali, Vanessa is using a straw to play around with her martini glass. In the next shot, the straw disappears.
Suggested correction: Daniel lives far away. Johnny must have driven all night.
That implies that Daniel lives like 5-6 hours from his job.
I actually assumed that it was already like 6am or something like that and knowing how sprawled our everything is there figured maybe it was like a 40 min drive. But then Daniel's wife also says it's Saturday morning yet Miguel and Sam are in school.