I told a dude who was signalling with his right arm bent upwards to indicate that he was turning right that it was wrong and potentially dangerous. He thanked me the next time and said he had no idea why he thought it was a thing.
I've yelled at people who are slowly weaving on their bikes while wearing headphones/earbuds and too spaced out to realize that they're blocking the MUP. They're more startled than thankful, but other people have thanked me for it.
Generally I keep my mouth shut if someone is doing it wrong, like wearing their helmet way too tilted up on their forehead or with saddle so low that their knees are splayed out like Charlie Chaplin when pedaling, because I'm sure they aren't interested in a stranger judging them. I've been tempted to remark that the law is to alert when passing and that I can barely see the ninjas riding without lights at night, but I feel like those are intentional enough actions that whatever I say would fall on deaf ears.