korien_oct8“You ate the pudding I left in the fridge, didn’t you? I even wrote my name on it!”
“I’m sorry! It won’t happen again.”

We say something won’t happen again when we have done something wrong and have to apologize. The other person can be sure that we won’t do it again, or won’t let it happen again. You can say this about something you did yourself, something a member of your group (company, family, friend, etc.) did, or something you failed to stop from happening.