Just trying to gauge opinion but I'm pretty sure I wasn't BU
Walking home from school this afternoon having picked DD up, casually strolling through the estate to get home, chatting with another mum and her kids as they absolutely love our dog when I notice a young boy who's in front of us but walking about 5 paces behind his mum and possibly his older brother, and he's spitting on the floor with every other step 
He's obviously got sweets in his mouth as the spit is bright red/pink and we're then having to dodge round it so we don't walk in it (gross) cos it'll be sticky and disgusting and I don't want someone else's spit all over my shoes and/or dogs paws
I said "ew can you stop spitting" in a jokey/normal tone, didn't raise my voice, wasn't angry or confronting, just wanted him to stop spitting on the pavement
His mum turned round and said "did you just tell my kid off?" to which I replied, "well, he's spitting all over the floor" and she responded by telling my to mind my own business and concentrate on my own kid - I responded by pointing out that my child wasn't spitting everywhere... again, didn't raise my voice, didn't get angry or animated but she kept chunnering all the way down the road till they eventually turned off onto their estate
Now, I'm sorry, actually not sorry in the slightest but if my DD had been the one spitting and another mum had pulled her up on it, I would have also told DD off for spitting, not the other mum for saying something
And seriously, spitting is fucking disgusting and she ought to teach her child some manners