This happened a few days ago, but I'm still thinking about it.
I was walking my DS to school, when a car drove past us - I could see there was a woman driving, and a boy in the back seat who was not physically restrained in any way. When I saw him, he was standing up, hanging between the front seats. Speed was no more than 10mph as it's a busy road at that time of day and the traffic was stop-start, but (for instance) a vehicle behind could easily have rear-ended the car and knocked the boy forward.
When we got to school, I saw them get out of the car and they then walked up beside us. I nearly didn't say anything, but then I couldn't bear not to 
I said "Excuse me, did I just see you driving down [road]? And your son wasn't strapped in to his car seat?".
She said "Er, yeah" and did a sort of back-and-forth hand gesture as if to indicate "but it's not far".
(I MENTION THIS PURELY BECAUSE IT IS GERMANE TO THE SITUATION
the woman was of east Asian appearance, so given the hand gesture I wonder if perhaps she doesn't speak English fluently).
I took a deep breath, already feeling stressed out by the confrontation, and said "I'm sure you know this really, but even if it's "not far", your son still needs to be strapped in every time. It's really dangerous for him to travel in the car unrestrained".
At this point, she looked grumpy, clearly decided she didn't want to continue the conversation and strode off ahead of me. Perhaps I should have let it go at that point, but I felt I needed to justify my interference in her life. So I shouted after her "I'm so sorry, I REALLY don't normally judge people's parenting, but you were putting your son's life at risk".
WIBU? I am swinging between thinking I was an unbearable interfering busybody, especially to talk to her right outside the school in front of a lot of other parents (but then, I had to say it when I saw her - I don't know her, and haven't seen her before or since), and thinking I don't care what I am as long as I made her think twice about strapping her son into his car seat next time 