Depends on what you wrote. If it says, "A gallon of milk and a loaf of bread" nothing will happen. If you altered the name of the medication or the amount, that's a crime.
In most cases, the pharmacy can report you for tampering with a prescription, if the information is pertinent. The date, name, dosage, etc. If this is a narcotic prescription the cops may be called without you even being warned. Arrest will most likely follow. My best advice is ask your doctor's office to redo or change ANYTHING on your prescription!!! It will save you from any unnecessary problems.