my previous dog was a german shepherd,attended obedience and training classes since 14 weeks old,was exceptional on and off the lead,at 16months old we went to our local park for his "run around" played with a few dogs,chased a few balls then he seemed to get aggressive with one dog in particular and chased it off,waited a few minutes for his return,but he never did..found out days later there was a bitch in season on the loose,despite posters in vet surgeries,supermarkets,even on the internet,we never got him back,so no matter how well you think your dog is trained,there is always "something" that will seem far more important.Our 8mth Husky gets off the lead surrounded by an 8ft fence.....and she has been trained too:(