Yeah you have to be so careful, they can be buggers!
I've only ever let Apollo of the lead twice. The first time he crashed through an electric fence to chase some pigs, the pigs stood their ground and he decided it wasn't worth the hastle and walked off!
2nd time was on a (what I thought) was a deserted beach in Wales, I let him off but in the very far distance Apollo spotted a boarder collie and took off after it, he jumped into the sea to catch the collie while it was fetching a ball, luckilly I managed to jump on him when he came out of the water