A promissory note can become unenforceable under the law if it is not written correctly.
If the note is not signed by the borrower, it becomes invalid.
If the interest rate exceeds the maximum limit under the state law, the note may become invalid.
If the note is altered without mutual agreement, it becomes invalid.
Notes with ambiguous terms such as unclear dates or missing party details can also become invalid.
Additionally, a promissory note is a contract, and just like any other contract, it should fulfill all requirements of a valid contract such as minimum legal age of the parties, soundness of mind, and legal objective.