I just put myself in that scenario and had the thought “man, what if she’s flustered about something else, and having a rough day in general, then I go and swoop in and basically accuse her of being poor in public. She’d feel terrible and I will have made her day worse.”

You would have walked out of there feeling like shit in two of the three ways that could have gone. Don’t beat yourself up over it; clearly you’re empathetic and care about people. That tells me you DO act when it’s appropriate and you do want to be a positive force in this world. Congratulations! The world is better with you in it