Med errors need to be reported at the time of discovery for many reasons, not the east of which is patient safety.
We had a Coumadin screw-up a couple of months ago that involved five nurses over three days. It was an order entry error that multiplied. When we (the floor nurses and I) discovered what had happened we immediately figured out the why of it. I then wrote a med error for every nurse involved in the chain so we would have the entire system flaw mapped out. I brought it to the DON along with our solutions to prevent it from happening again. Within two days there was a new policy implemented and some of the people involved, specifically the LPN who discovered it and came up with the new system, were commended for their root cause analysis. No one was "written up," nor was it necessary to contact the originating nurse prior as we figured out how and why it happened.