Usability testing aims to evaluate how users interact with an application to identify areas for improvement, but it is often marred by common mistakes that can hinder its effectiveness. Key pitfalls include inadequate planning, misconceptions about the test's goals, choosing incorrect participants, and conducting tests too late in the development process. Additionally, issues arise from one-way testing, interruptions of participants, conducting only a single testing phase, and skipping essential pilot tests. Proper task design, testing potential solutions, and effective facilitator management are crucial for successful outcomes, as is treating participants with respect to avoid premature conclusions. These errors underscore the importance of thorough planning and learning from past mistakes to ensure usability testing contributes positively to the development process.