Ive had just one close call over my smoking 'career'. This was a few years ago, when Herb was a class B drug in England (Schedule II to all you Yanks).
Anyway, I picked up a nice, fat eighth from my mates and I felt obliged to smoke them up. One of them had bought a bluntwrap with them, and I had never had a blunt before, so we packed it with half of what we had. Damn it was a fat blunt! Anyway, we were sitting in the park, about 10pm smoking it on a bench/seating thing, when a cop stands at the gate to the park that was about 15-20 metres away. He was also downwind of us, so Im sure that he knew what we were smoking. We looked at him, and he just stood there staring at us. We carried on smoking, and it mustve took us about 30 mins to smoke this thing between us. We were fucked. So we throw the roach away, and walk out of the only gate... the one that the cop was standing next to. As we walk past, he just said 'alright lads' and we just replied 'alright officer!'.
He walked away in the other direction. Something I learnt about cops in England...

They dont give a damn about you smoking in England, as long as you arent causing trouble.