A notary should never, under any circumstances, notarize:
Notaries can notarize signatures of business associates if they are present and properly identified.
Judiciary Law § 135 and the long-standing rule on acknowledgments prohibit a notary from acting where they are a party to the instrument or otherwise personally interested; a notary may not certify their own act. New York authority states that a grantor cannot take his or her own acknowledgment, so notarizing one’s own signature is void as a matter of law.
Notaries can notarize documents from other states if they comply with local laws.
Notaries can notarize signatures of close relatives unless prohibited by state law.
Explanation
A notary cannot act when personally interested in the transaction: “if the notary is a party to or directly and pecuniarily interested in the transaction, the person is not capable of acting in that case.” A notary signing their own signature would be notarizing their own act, which the source says is barred because “a notary who is the grantor could not take his own acknowledgment.”