It's definitly not common, but it happends.. when a pharmacist is busy mistakes can be made .. now, look on the bottle - ar the pharmacists initails on the bottle? That means it was re-counted for accuracy, this is common practice for controlled substances .. now, if it's a controlled substance they will count down the stock and see if they are short - because EVERY pill is accounted for, they can count down what they have left and see if there are any extras - this was common when some junkie would come in with his narcotics script, walk out to the car, then walk back in and claim we shorted him a few - we would count down stock, find out we have no extra, and kindly tell him to get out of the pharmacy .. this isn't like scamming a meal from burger king, you can't just be persistant and expect them to make up the difference - though sometimes they will if you're an older adult and you are clearly not an abuser..
There is always the case of pharmacist diversion, but it's a hard thing to prove.. sorry, you're not in good shape to win the case of "the pharmacy shorted me" - if it's C-II you can forget it!
How old are you? If you even say you're over 18 I am going to ... well, I will do something, and it wont be good! No adult should regulate your medication, how the hell are you supposed to abuse it that way?