It can happen. I have a good, wiry working Lab, that is also a pet. Normally he'll work quite happily all day, generally doing a bit of hunting/beating but with some drives being a peg dog (much to his annoyance). Last season due to a few coincidences, he was the only dog available to work. I try to pace him, but they will go at full speed all the time. Eventually he did flake out. A bit of sustenance and a bit of time and he was off again! Yes, chocolate in any quantity is not good for them, but if all you have is a chocolate bar in your pocket when the dog needs a shot of something, it's better than nothing. I always now carry something non-chocolatey.
Just as an aside, on Saturday I sent him to retrieve a shot snipe. Off he went and came back, I thought with nothing. But lo and behold he presented me with it. Had been wholly in his mouth!