Validation Rule

A validation rule is a formula that must evaluate to false for a record to save; when it evaluates to true, Salesforce blocks the save and shows the rule's error message. Rules deploy as metadata and take effect immediately once active, there's no grace period, so a validation rule deployed against production data that violates an assumption baked into the formula, a null field, an out-of-range value from years of legacy records, can start blocking saves on existing records the moment it goes live, even though it tested cleanly against clean sandbox data. Validation rules also fire on every save, including from Flow, Apex DML, and integrations, not just the UI, so a rule that only accounted for manual entry can unexpectedly break an integration or batch job that never triggered it in testing. Our change sets to continuous delivery playbook covers rolling out rule changes safely.