as a formaer pharmacy tech I know from experience that mistakes are made.
we all know that peoples lives are on the line when we work in a pharmacy, however, we are humans, and we do make mistakes, especially when there is an irate customer at the register complain about her copay, the phones are ringing off the hook, half the tech and pharmacists are on the phone trying to authorizations from doctors (and fix their mistakes!) and prescription plans, you have 20 people waiting and they all want their meds right NOW!
I know that these are not excuses for a mistake being made, but these are factors in loss of concentration.
You should go back to your pharmacy and tell them of the mistake, and that you are not pleased. They don't need a "scare", they need to know that they made a mistake so they can correct the problem.
From now on you should also check your meds, even before you leave the register, if it looks different than you have had before speak up. I am surprised you didn't notice the difference before you took the meds. Paxil is a table, Prozac is a capsule and are completely different colors.