Mine was years ago, back when I was a bit of a goon.
Me,
@dansgas1000 and about 8 others all made the trek down to Essex one Friday night to look at the blood bank. Me and one other matey ended up driving, it absolutely hammered it down with rain all the way there, proper heavy rain.
We arrive at the area, driving about looking for suitable parking, found a lovely big empty car park and got out to scope it out on foot. Walked round the back of this massive building trying to find a way to the blood bank. Couldn't fathom it so walked out onto the main road.
Heard sirens in the distance, a police car came flying up and stopped right by us, got out and started questioning. Then another, and another, then a van. In the end I think there were 6 police cars and a van (and possibly a dog unit if I remember correctly) all in attendance.
We had only gone and parked in the grounds of some sort of active medical research facility which, just the day before, had travelers trying to break into so they were on incredibly high alert. Funnily enough they don't take kindly to a group of Norfolk yutes poking about their facility at near midnight.
They were understanding but it didn't go without a full search of my car, taking my name and checking me on the police computer ect.