Debit Card Dispute - Reg E or Not

Ok, this is a really basic stupid Reg E question so I apologize (I usually only work on fraud). A customer is disputing a charge made with her debit card because she said the merchant did not complete the transaction and she ended up paying cash. My initial thought is that this is unauthorized since the transaction wasn't supposed to be completed, but another part of me thinks it doesn't fit the definition because it was initiated by her. But isn't it still an error and have to be investigated? Any insight would be appreciated.

If the transaction was not completed and she paid in cash, it sounds unauthorized to me.