I said if I was smart that I would save up for a piece of string, and a rock to wind the string around

Sometime in the past month or so, a two cent coin found its way into my wallet. I have no idea how it got there. Then there is the question of why I've left it there so long since I first noticed it; but then that's just classic wallet inertia. It seems to belong ok with the three half-used sets of stamps and the old video cards.

But how did it get there? Did some cashier give it to me instead of a 5c coin? This bugs me. Is there any other way I got it other than being ripped off by three cents? (suggestions please).

Oh yeah. For the benefit of any non-Australian readers:
The copper coins, 1 and 2 cents, were phased out of our currency several years ago. Although the smallest coin is now 5 cents, prices are often still not divisible by 5 cents, especially in supermarkets. The total is rounded to the nearest 5 cents.