If you use PayOne as a payment provider for your website and you run into a message that says the hash value is incorrect, here’s a little secret.
PayOne has an extra parameter you can add to their webservice, which is called testhash with a value of 1.
https://secure.pay1.de/client-api/?aid=xxx&mid=xxx&firstname=….&hash=MY_GENERATED_HASH&testhash=1
This sends you to a clean interface where you can see all your parameters, with the EXPECTED hash and YOUR hash. All you have to do now, is read the documentation again carefully and make sure they match (because you can’t go back from that hash!). 🙂