sound as though you had had what is called "cold fouling" and what some are talking about above, "cold fouling" is a modern phenomena, due to the fuel we now have to suffer, if you start your bike, but do not fully warm the whole thing through,( and riding off is the best ploy as Lowther states), then you can get cold fowling, it can ruin the plugs, but not always, the fuel leaves a residue on the plugs electrodes which cannot be seen with the naked eye, and is difficult to eradicate, new plugs does it, it wont occur if you do run the motor up to full temperature (almost) every time.
CLEM