Ok careful here because it does matter in certain circumstances..

You're on a date the woman points out something embarrassing like spinach or a black eyed pea ..stuck on your left frontal incisor..making you look like a pirate junkie....

Well there goes the positive mood the confidence..the conversation..self esteem..and possibly the date...

I once got scolded for not saying something. Now I always say something and it's always appreciated

True, but if you are a guy and see another guy with his pants unzipped, its always uncomfortable telling them. On one hand they are thankful, but on the other hand they are thinking, "are you looking at my junk?"

One time I was doing a first aid course and the woman sitting next to me was wearing a button up shirt. One of the buttons on her shirt was undone, and you could clearly see her bra.

I felt a little uncomfortable with telling her because I didn’t want her to think I was a creep and looking, but I figured, it’s obvious, and if she notices and realizes nobody told her, she will probably think everyone was staring anyway.

So, I subtly wrote her a little note to tell her and she fixed it, and thankfully, she was very appreciative and didn’t seem to think I was a creep. whew

All depends, if you like that person or not, he,he.

Would it not depend upon your relationship with the person, or if you think you could bring to their attention the situation? I have been able to discreetly tell someone that their fly was open, or there was something on their face/nose/whatever, and not unduly embarrass myself, or the person! I would rather someone tell me if something is amiss, than to find out at home later that I've been flashing lace or bodily functions all day!

I would want someone to tell me, so I would absolutely tell them.

You bet, afterall it is about me. ☺

some people are insecure but it's polite to call it out (discretely)

Just spit it out... hey .... ho look in the mirror.

That reminds me of that one scene in the movie The Emperor's New Groove! I don't know, I think most people won't tell you out of being polite but if someone told me I had eye buggers or something in my teeth, I definitely wouldn't think they were assholes.

Interesting. I tend to call that stuff out and a lot of people think I'm an asshole. Though I suppose that could be attributed to other factors as well.