Recall Issues or Failing to Return
In terms of training related areas that can cause worry and frustration, it must be the lack of recall which tops the list. The best time to set up sound recall practice is with the first outings with your puppy. Dogs that do not recall (some may actively run away!) present a danger both to themselves and the unsuspecting public such as a motorists on a public road.
As the responsible owner you are ultimately liable for any such accidents and their outcome, so you can see how the simple act of not returning when called can have multiple and potentially severe consequences. Non recalling dogs are also in my experience more likely to become involved with aggressive behaviour once they are beyond your immediate control.