1. Unsigned or invalid signature: the issuer of the bill has no signature, or the signature is invalid, such as the signature does not match the signature in the bank document.

2. Unknown payment reason: the payment reason on the bill is unknown or vague, which makes the bill not binding and cannot be used as a valid proof of payment.

3. Fraud or misleading: the bill is fraudulent or misleading, or some facts are concealed from the endorser or holder, resulting in the bill being deemed invalid.

When the bill has the above three conditions, it will be regarded as invalid and cannot be used for payment or other legal purposes. This is one of the basic principles in the bill law, which ensures the authenticity and legality of bill transactions.