Let's Talk Tact

Working in a retail environment allows me to come in contact with many different types of people. For awhile now I have wanted to right something about tact and more importantly the lack there of.
My friend and co-worker Mary made the suggestion months ago, but I just never had the motivation before today.
Another of my co-workers recently had her hair cut and it was cut very short, much shorter than she wanted. Needless to say she was very unhappy with the results and was trying to make the best of the situation. Before I go on let me say the haircut is not that bad, it is short, and she definitely doesn't like it but it's really not that bad.
One of customers, who we see quite regularly and chat with, made some completely inappropriate comments. Things like "thats a horrible hair cut", "I hope you didn't tip her", "It's not wanted you wanted, is it?".
I could go on.
I was dumbfounded that any adult would think these were appropriate comments to say to anyone else.
I don't think she said these things to be deliberately hurtful, but I'm not sure. The old adage "If you have nothing nice to say then don't say anything at all" certainly comes to mind.
It started me thinking about whether or not people are just plain stupid and don't think about what there saying or are the just oblivious to the effect that their words can have.
I still remember a few years ago I bumped into an old neighbor of my mom.
Now since I last saw her I have gained some weight.
I know this, it's not a secret.
What does she say?
"Oh, I almost didn't recognize you put on so much weight."
Now how do you answer that?
My first thought was Oh, I almost didn't recognize you either you got so old.
Now of course I didn't say that, because I like to think that I have more tact than that.
I would love to hear your stories, and whether you think some people are just being mean or do they just not care.
As for my co-worker with the newly short hair. She just smiled and said that the customer was just telling the truth.
Now that's tact.