not a karma story but somethings I believe are true.
Always say hello to beggars in the street, you don't know why they're there and never know if it will be you some day.
Always say hello to people on reception and the security types you never know when you will forget your pass at your desk and need to be let back in without doing a massive song and dance.
Always say hello and be polite to the cleaning staff - they literally have to put up with shit and are working very hard for very little money.
And as for the stories some have been relating about someone being ratty when the story teller was having a bad time....
About 20 years back my dad was in very poor health and I didn't get to see him as he was dying as I live in the UK and he lived in Canada. So eventually he died and I needed to arrange to go to be with my family.
A very jolly chap I worked with rang up to arrange a meeting and I had to say no I wouldn't be able to make it as I'd be in Canada. Jolly chap pipes up "oh lovely going to have a nice time visiting with everyone and slapping mounties on the bum?!!" My reply was "no it's for my dad's funeral" and burst into tears. He was incredibly apologetic and spent about 10 minutes saying "oh sorry, I'm so sorry, I didn't mean to upset you, So sorry".
I know he meant it and quite frankly I almost started laughing he sounded so upset himself.
Personally when we react to these things I do think that the vast majority of people who've said "smile" or whatever would love to sink into the ground or do whatever they could to make you feel better or take back their words.