I've had a med error while a nurse, never in EMS though. I gave Fioricet instead of Fiorinal (GASP!) the patient was fine, the provider wasn't upset but I have seen some tragic medication errors. One nurse gave 160mg of Oxycodone to a patient and it had the potential to be tragic. I also know of a nurse who didn't call in a PT/INR and the patient died of DIC.. pretty extreme.
Most hospitals I know have a non-punitive response to med errors to encourage their reporting and monitoring. There have been several safeguards that I'm not sure would work in EMS. After my medication error, I slowed my med passing down a bunch, took time to double check the med I was pulling against the MAR and review the 5 rights.