It's a scenario that many of us have been in. Your dog is a rockstar in training at home. The cue 'sit' has his back end on the ground before you can blink. Down sees him flat on the ground, looking at you attentively. 'Stay' results in a motionless dog, concentrating entirely on you as he waits to be released.
Okay, so maybe those might be a little exaggerated - most of our dogs are, after all, works in progress - but let us say that your dog has got a good handle on basic cues in the house and garden.
You take him out for a walk and meet up with some other people in the local off lead dog friendly fields. You see something up ahead and think that you'd like to keep your dog back for a few moments while you have a chance to make sure of what's going on. Confidently, you instruct your off lead dog to sit. Your dog, meanwhile, has also seen something up ahead and is merrily frolicking off to discover what it is. You turn to your friends and say 'I don't understand, he does it every single time at home!'