Medication Error, What do you think

Ok I need some support from fellow nurses. I made a med error last week it was a mistake and I felt so bad. I work in home care and the pharmacy delivered the wrong med to the pt. the pt was ordered to recieve Ancef but the pharmacy sent Rocephin and the med was given to the pt by a family member but I was present at the time. The pt was not hurt and had no reaction to the med Ok so what's the problem?

Well this is the first time in my 4 years as a nurse that I have given someone the completly wrong med and I have never been under any dicipline before at work. Well my supervisor who is a pharmacist, gave me a final written warning! so if it hapens again I am fired! I told her that it was unusually harsh to give final written on a first offense but she stuck to it. I filed a grivence and the people in HR (none of who are nurses) all felt like that was appropriate.

I am at as loss here! as soon as I realized there was an error I contacted the pharmacy and I have been honest about everything so why this baptism by fire?

none of the people in administration are nurses so I feel like I am all alone without any support from fellow nurses who have been through med mistakes.

Can anyone share experences with med errors and maybe the action their supervisor took? I feel like I am being singled out by my manager and being punished for mistake.