*snort* That happened to Theo more than once when he was a puppy. Would do a great job for 30 minutes, and then run into the center of the ring and let loose. Remember, Colin, that's why you wanted a young dog--for the fun!
visualise this... I was demo-ing the sit-stay command with a young boxer, normally a whip-smart cracker-jack dog, in front of a full class of people.
Bring the dog out, give the command, the boxer stands there wagging happily.. repeat the command, and, in front of a silent class of dog owners, you can hear a pin drop.. or a teeny tiny boxer fart "POOT"
followed immediately by a proper sit, and a class room in hysterics.