I think for me it was back in college where I thought it would be funny to laugh at someone in my class for not being able to read/speak English or French.
The class froze and the girl answers back to me saying "It is not funny"
I kindly said sorry but she hated me.
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Oh man I've done a lot of immature things. One thing: I once convinced my whole class to hate one girl because she had been allegedly cheating on my friend. My assembled team made her break down and cry, we were on the swing sets listening to her sobbing. We found out then that perhaps one friend's feelings aren't worth hurting another's.