Firstly, we see Ranbir and Katrina Kaif eyeing each other as she dances with a drop of aunties. Then when we see more of Ranbir, you can see that he is wearing some sort of uniform. The next shot shows a yellow school bus, giving the impression that this is a school setting. That Ranbir is a student, and Katrina is a teacher.
This is confirmed when he is then accompanies by a large number of kids in uniform dancing hysterically. The problem though is Ranbir is a definite oddball in the school; and due to his height, stands out a bit more.
Throughout the song, the kids are enjoying the song and dancing along with the tune. Though there is no reference to what the ‘mistake’ is that they are referring to. It may have something to do with Katrina, as none of the kids a centred out of the song.
See the full music video here: