Mine was nearly 20 years ago when I was driving and went to let a car into the junction. It was my best friend's mum! She waved, and for reasons I struggle to understand today, instead of waving back, I flicked her the V's 
which is something I NEVER do when driving, I have no idea where it came from. I had to call my friend (on the house phone of course) as soon as I got home and say "Tell your mum I honestly had no idea why I swore at her just now" 
we all laugh about it today thankfully