In secondary school (Northern Ireland) it was rumoured that one of history teachers, had a massive tattoo on his back that associated him with a loyalist paramilitary aka a murderous criminal gang (the protestant, non security forces side of the troubles).

This was in my generation, but people I knew outside of school that had attended this school, and were around 10 years older than me, and people I knew that went to the school and were 7-8 years younger than me that also went to the school, had also spoken of it. Whether there was any truth in it or not I'm unsure, but it was definitely passed down through generations. What I always wondered was who would have seen him topless for the rumour to begin.

Another, we have a soft drink made locally by Coca Cola NI and in the Republic, it's called BPM. It was 35p for a 500ml bottle when I was a kid, and it stayed quite cheap, so it's price made it very popular amongst kids. When I was a kid, around 2001, a rumour started "if you drink BPM it'll make your balls shrink" even now, people of my generation make reference to this rumour, and if someone was seen drinking a bottle of BPM at school they'd have got "yano that makes your balls shrink?" Viscous rumour and nobody had ever concocted a rumour of what it done to girls.