Sometimes you need to criticize, critique, or correct someone.
A question to ask yourself that I got from Brian Sharp before starting:
Ask, "Am I communicating in a way that's going to be helpful or harmful?"
Bringing the right intention doesn't always lead to good results, but it does almost always lead to better results than you'd get otherwise.
It's easy to let one's own frustrations spill out when things go wrong. But that's communicating for your own sake, to gratify yourself, and not to actually help and do right.
If you have to have a tough question, check your intention first. Ask if what you're communicating is meant to be helpful, and if it will be perceived that way by the other person. If you get that truly right, finding the words becomes much easier.