Not Too Soon, Not Too Late
When a problem happens, you should address it, but only when the time is right.
If you are the kind of person who reacts emotionally first, take a moment to count to 10, take a break or sleep on it.
If you can address a situation immediately without loosing your cool, go ahead and address the issue. The longer you wait, the less effective you'll be.
Ask Permission
Let's say you observe some behavior mistake someone made, or a situation that they mishandled. Before you begin to offer your feedback, ask. If they say 'no' respect that and drop it.
Feedback is given to help people create behavior changes. If they're not open to coaching, don't push the issue.
Share Understanding
If you have a misunderstanding, the best way to clarify the issue is to approach it in a non-confrontational way.
Something like "In our agreement, I understood that this would happen. But it did not, and I'm wondering why. What was your understanding?"