This wasn't me, but I had a colleague who noticed that a woman had refilled her husband's Nitrostat several times in the past few weeks. He asked her about it, and she said, "I have a hiatal hernia, and I found out that this works for me." 😱 He told her that this was probably not a hiatal hernia and to see a doctor that day, and about a week later, she came in with a pile of RXs from a cardiologist in the nearest big city.
I had a faxed prescription from a pediatrician who ordered something (I don't remember what) for a small child, and it seemed like a really big dose. The mom wasn't happy that she had to come back later, but it turned out he'd entered "teaspoons" instead of "milliliters" into the computer, and told her that if we had not intervened, it could have killed the child.