To design an effective error message in a GUI, follow these best practices:

1. Clear and concise language: Use simple, easy-to-understand language.

2. Specific error description: Clearly state what went wrong.

3. Relevant error context: Provide context about the error.

4. User-centric tone: Use a friendly, non-technical tone.

5. Visual hierarchy: Use headings, colors, and icons to draw attention.

6. Error prevention: Explain how to avoid the error in the future.

7. Recovery options: Offer solutions or alternatives.

8. Minimal technical jargon: Avoid technical terms or codes.

9. Consistency: Use consistent error messaging throughout the GUI.

10. Accessibility: Ensure error messages are accessible for all users.

11. Timely display: Display error messages promptly.

12. Dismissal option: Allow users to dismiss the error message.

By following these guidelines, you can create effective error messages that help users quickly understand and recover from errors, improving the overall user experience.