I've heard another problem is the similarity of names for prescription medications. Doctors are already notorious for their bad handwriting, and sometimes a pharmacist will misread the prescription. I think they have better safeguards in place now, but I remember hearing stories about patients being prescribed a medication I'll call Problovia when they were actually supposed to get Provobilax. The names are so similar that an occasional mistake is almost inevitable.
I think doctors today allow their assistants to submit the prescription electronically to pharmacies and any discrepancies or interactions are supposed to be flagged ahead of time.