A strange part about being human is that we have all been petty at least once in our lifetime. And when it comes to your ex, sometimes, we can simply be extra petty, more so than necessary, just like Reddit user OnionVinegar, who felt he took things too far.
His ex-wife had a list of food to not give their 10-year-old, and it included onions as she felt that they would make their daughter smell bad and she might get bullied for it. But when she found out that he had given them to her, she threw a fit!
More info: Reddit
