If the data is invalid, it will return a nice and clear error, indicating exactly where and what was the incorrect data.

It seems like you're asking how to handle invalid data in a program. Here's a general step-by-step guide:

1. Input Validation: The first step is to validate the data as soon as it is received from the user. This can be done using various techniques such as regular expressions, built-in functions, or custom validation rules.

2. Error Handling: If the data is found to be invalid during validation, the program should handle this error gracefully. This could mean stopping the execution of the program, or it could mean skipping over the invalid data and moving on to the next piece of data.

3. Error Message: The program should return a clear and informative error message to the user. This message should explain what the error is, where it occurred, and, if possible, how the user can fix it.

4. Logging: It's also a good idea to log the error in some way, so that you can review it later. This could be as simple as printing the error message to the console, or it could involve writing the error to a log file or database.

5. Recovery: Finally, the program should attempt to recover from the error and continue executing. This could mean asking the user to input the data again, or it could mean using some default data in place of the invalid data.

Remember, the specifics of these steps will depend on the programming language and framework you're using, as well as the nature of the data and the requirements of your program.