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

What common mistake do developers make with Thin controllers, why does the mistake cause problems, and what is the safer approach? One recurring problem with thin controllers is applying it without understanding the boundary it protects. Putting database queries, validation, notifications and complex business rules into one very large controller method. In addition, Moving complex business logic into services or domain classes can keep controller methods easier to read and test. 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.