I know its an oft-uttered fume, but people who drop cig butts. Especially the ones who just do it so naturally, so absent-mindedly, so assured that there is absolutely no problem in doing it. Saw some otherwise-respectable looking lady do it this morning; final drag, then straight out of mouth and onto floor in one smooth swing of the elbow. Didn't even stamp it out.
I just don't get why people still think this is acceptable? You honestly wouldn't do it with anything else. Even chewing gum, most people will look for a place to dispose of it. There are so many grills to dispose of cigs now, there is simply no excuse.
Could always say summit to her in the street, she is likely to be embarrassed by it and unlikely to do it again for the fear she might get caught again.