What common mistake do developers make with Conditional validation, why does the mistake cause problems, and what is the safer approach?

What common mistake do developers make with Conditional validation, why does the mistake cause problems, and what is the safer approach? One recurring problem with conditional validation is applying it without understanding the boundary it protects. Validating a value but then updating the database with the complete unvalidated request payload. In addition, Validation can change based on other input values or business conditions. Safer engineering approach: keep the responsibility in the correct layer, use the framework's supported API, validate and authorize inputs, avoid hidden side effects, and verify the behavior with tests.