In Manchester in the UK there was one in 2019: an old railway line which has been converted into a bike path, the Fallowfield loop. Groups of teenagers were attacking cyclists with hammers and logs. I believe that the police woke up to it and it has calmed now but it was definitely a no-go area for me for a few months. I'm pretty sure someone tried to drop a fist-sized rock on me while I cycled under a bridge...