There are a few key mistakes that should be avoided in user interface design:
1. Clutter and complexity: Users shouldn't become overwhelmed with information or have trouble finding what they need.
2. Inconsistent design: Any disparity in colors, fonts, or layout may cause users to become confused.
3. Lack of Accessibility and Poor Feedback: Not everyone has equal abilities, making it essential to create interfaces that are usable by all.
4. Poor Feedback: Users require clear confirmation or error notifications after taking actions on websites and applications.