Mmm, pears. Oh, sorry, we got distracted for a minute. So, one of the first real sins that Augustine commits—aside from throwing tantrums as a baby, which he can't remember—is stealing pears from his neighbor's tree. As a result, the pear tree came to be a symbol used over and over again in medieval literature. Can you think of another "original sin" that involves stolen fruit? That's right, we're talking about the Forbidden Fruit episode in the Garden of Eden from Genesis.
A lot of people might wonder why Augustine makes such a big deal about thieving some fruit, but this act marks an important moment in Augustine's life as a sinner. This is the first of his sins that he can remember. This is the beginning of it all. Sure, Augustine sinned before that, but this is the first time he consciously sins simply for the sake of sinning… and he does so for no apparent reason. He's not even hungry, and he doesn't eat the pears. So this thievery is actually a pretty sinister event, especially considering all of the other sins that Augustine is going to get into soon after: like sex, Manichaeism, and pride. You guessed it: stealing pears was, like, Augustine's original sin.
Speaking of, let's get back to what we were saying about the Garden of Eden. According to Genesis, eating the forbidden fruit was what caused everything to go awry in the world. It was literally the first sin from which all other sins were born, even though it, too, seemed like a minor thing. The point is, in both instances Augustine is showing that there is an impulse in us to sin—Augustine tells us that "If any part of one of those pears passed my lips, it was the sin that gave it flavor" (II.6.1)—and that it is something that has to be actively combated. He's also saying that even in the most innocuous sin are the seeds of greater sins to come, so watch out: it's the little sins that'll getcha.