Around the 52 minute mark Lee meets Marco, when he does he has two crutches, soon after in another shot there is only one crutch.
7 mins 49 seconds in when the school secretary goes to Elles class to say that she has a phone call, Elles desk has lots of stuff on it, school books, mobile phone, pencil case and jumper, then a second later the camera goes back and she only has a pad and pen then another second later when Elle goes to stand up, the desk has nothing on it at all.
When Noah meets Elle at the airport and tosses the sign, it hits an extra in the back, who flinches.
In the first The Kissing Booth, Joey King's character is listed as Shelly "Elle" Evans. But at the end of The Kissing Booth 2, Noah calls her Rochelle.
The second time Elle and Lee notice that their high score had been broken, the view from inside the screen shows MVP's score as 58525 but when the score is shown from the front view it reads 48244.