So, you gave a rubbish dou cut on someone's rib. You must've been in the reverse situation too.
This is the kind of unintended 'give & take' you have to accept in kendo practise.
You'll quickly learn that you do not have control over everything, and sometimes you are forced to give or take things you don't necessarily want to trade.
By giving someone a rubbish cut, you'll learn to be humble about yourself. And by being the receive end of it, you'll learn to be unmoved by adverse situation.
When you know you missed the target, briefly acknowledging your fault is advisable, but when you received a bad cut, you are not supposed to show your suffering or get angry.
If the bad cut is deemed intentional or outright dangerous, you should halt the practise and calmly express your concern to your partner, but never take it out.
It might not be the same person you do 'give & take' but, there's always unintended give & take happening in kendo practise, and that is an opportunity for you to learn how to deal with it in a calm & constructive manner.