In such a case any pharmacist can generate a factious certificate. I don't expect them to risk their licence with false data entry but given a big country and a sufficient number of disgruntled, nearly retired professionals...
[0] https://www.br.de/nachrichten/bayern/hunderte-digitale-impfp... (German)
https://cyprus-mail.com/2021/08/14/coronavirus-new-remand-fo...