Quote from HIPAA
I'm not a lawyer, but I'm fairly certain that you can file a lawsuit against a previous employer for giving out information other than dates of employment and performance reviews.
I may be wrong, or it may be state to state.
It's certainly possible that a med error could be noted in a performance review, particularly if it had serious consequences or was a pattern rather than an isolated event. Also, the person calling for the reference may ask if the employee is eligible for rehire. A response of "absolutely not" is pretty telling. And really, if I'm calling for a reference, I would certainly want to know if the employee was a serial absentee or a danger to patients!
To the OP, I seriously doubt there is a nurse in the field who has not made a medication error at some point in their career. If they say they haven't, I'm thinking either they are not being truthful, or it just wasn't picked up on , so they didn't know about it. No one is perfect.
Where I worked, all medication errors were written up. The point was not to be punitive, but to address errors with the purpose of figuring out how they happened so that they could be prevented in the future. It's all about patient safety and being able to learn from one's mistakes.