Invalid signature
From Ballotpedia
An invalid signature on a petition is a signature that has one or more flaws such that it does not meet the legal standards set forth in the jurisdiction in which the signature was obtained for a legal, valid signature.
After a ballot measure campaign or a candidate campaign has turned its signatures into the government agency--which could be a county clerk, an elections office or a Secretary of State's office--which is the designated agency in that state for determining the validity of signatures, that agency will begin the process of scrutinizing the signatures to determine their validity.
However, like the famous Florida hanging chads in the contentious U.S. presidential election of 2000, when the workers in charge of signature certification examine an individual signature on a petition, it is not always crystal clear whether or not the signature is valid.
Additionally, different states have different norms or attitudes about what should count as a valid signature. Some jurisdictions historically follow a "If in doubt, throw it out" attitude, and some jurisdictions, by code or by traditional norms, have a "what the signer intended is what matters" attitude.
