My question is kinda lengthy here. So, the important question is regarding a situation.
So, there was this guy who bought a wrong book, since it had the same name but the book had a different number on it. Everybody was laughing, not as insult though, just because the situation was funny when he realised he was doing the wrong sums from the wrong book. So, when everybody was commenting on it, someone said, obviously as a joke, "can't you read numbers?" and laughed it off. The guy who bought the wrong book obviously didn't mind and now, he probably doesn't even remember about that remark since everybody was talking. Now, it is not even a valid insult because it has nothing to do with him not being able to read numbers. He didn't specify the book number and that's how he got the wrong book.
The insult is kinda like jokingly calling Albert Einstein stupid because he forgot his pen or something. And the person who "insulted" the guy didn't mean it as an insult at all. In fact, she didn't even know insulting someone was a sin, but she was getting bad vibes about that action but wasn't completely sure and then researched it, and she found out it might be a sin.
Now my question is: " Is it a sin in this case?"
It would be great if you could quote hadiths and passages from Quran where it says it's a major or minor sin. But my main question is regarding the situation above.