Using my tri-blade yesterday I hit a mains water pipe going into the little house I was working at. 16mm hard poly pipe going to the ground, above that was all copper. Easily fixed though, get a proper 16mm plumbing joiner, cut pipe and install joiner.
Best thing though, It was my inlaws place, and only cost about $15 to fix.
Why would they run poly to the ground and not copper into a elbow then poly? stupid I think.