Here are some common mistakes that can degrade the user experience in a GUI:

1. Poor navigation and information architecture

2. Inconsistent or unclear typography and labeling

3. Insufficient or confusing feedback and error messages

4. Overly complex or cluttered layouts

5. Inadequate contrast and color choices

6. Non-intuitive or inflexible interactions

7. Slow performance or responsiveness

8. Lack of accessibility features

9. Inconsistent or broken branding and visual identity

10. Ignoring user testing and feedback

11. Overuse of modal windows and pop-ups

12. Poorly designed or missing tooltips and help resources

13. Inadequate search functionality

14. Unclear or missing calls-to-action

15. Not adapting to different screen sizes and devices

These mistakes can lead to user frustration, confusion, and abandonment. By avoiding these common pitfalls, designers can create a more intuitive, engaging, and user-friendly GUI.