Give an Example of a Simple Invariant for a Token Contract

A simple invariant for a standard ERC-20 token contract is: "The total supply of tokens must always be equal to the sum of all individual user balances recorded in the contract." This property must hold true at all times. If a transaction causes the total supply to increase without a corresponding increase in a user's balance, or if a user's balance changes without the total supply or another user's balance changing correctly, the invariant is violated, indicating a critical bug like an unauthorized mint or a logic error.