The problen in this sentence is redundacy. The word "accidental" placed in the sentence to refer to a mistake the pharmacist made, is redundant since if it was a mistake it is evident that it was non intentional, so that it was an accident.
Redundancy is usually defined as the use of two or more words that say the same thing, but we also use the term to refer to any expression in which a modifier's meaning is contained in the word it modifies.