Was the sale a demo sale?

The MD5 hash most commonly fails because the account is in demo mode. If the sale was in demo mode, then the order number used to compute the hash on the 2CO side is forced to ‘1′. This will cause the hash to fail since the strings will not match up. This is done intentionally to protect those who sell digital goods from fraudulent purchases.

If the hash fails and was not a demo sale, then this most likely means that someone tried to access your digital goods without paying for them. That is exactly what the MD5 hash is designed to alert you of.

If the hash is failing on a INS post please read the related article below for more information about the MD5 specifications.
How do I use the MD5 Hash?