Although this episode aired in 1998, it was made in 1997 according to the credits.
Dexter has apparently never farted before. Which explains, along with his youthful gullibility, why he thought a balloon popping from overfilling it with air would be his fate.
Dexter should have let out a fart a lot sooner than his watch predicted, or at least needed to use the bathroom. Overindulgence of food often leads to the need to use a bathroom to make room for digestion.