Common mistakes in usability testing frequently undermine its effectiveness. Ignoring critical results (ii) negates the purpose of the test, as identified issues are not addressed. Scheduling the test too late (iv) in the development cycle often means there is insufficient time or budget to implement necessary changes. An incorrect focus (v), such as testing with the wrong user demographic or on irrelevant tasks, leads to invalid or misleading findings. These three points represent significant and widely recognized pitfalls in the practice of usability testing that compromise the value of the exercise and the quality of the final product.

i. The user is too active: An active, engaged user is desirable as they provide more extensive data and insight into the system's usability, making this a positive attribute, not a mistake.

iii. No observers involved: While having observers is beneficial for stakeholder buy-in, their absence does not invalidate the test itself, provided the sessions are properly moderated, recorded, and reported.

1. ISTQB® Certified Tester Foundation Level Usability Testing Syllabus (Version 2018).

Section 3.1.1 (Formative Evaluation): Emphasizes that formative tests are conducted "during the design and development process to find and fix usability problems." This supports the idea that scheduling tests too late (iv) is a mistake.

Section 4.1 (Developing a Usability Test Plan): Details the necessity of defining the "profile of the participants" and the "tasks that the participants will perform." An error in these areas constitutes an incorrect focus (v).

Section 5.3 (Communicating Usability Test Results): Stresses the importance of communicating findings to stakeholders to "facilitate the fixing of problems." The failure to act on these findings, i.e., ignoring critical results (ii), is a primary failure of the overall process.

2. Dumas, J. S., & Redish, J. C. (1999). A practical guide to usability testing. Intellect books.

Chapter 13, "Avoiding Common Mistakes," pp. 337-340: This chapter explicitly lists "Usability testing is done too late to be effective" (iv), "The sample of test participants is not representative of the user population" (a form of incorrect focus, v), and discusses the problem of results not being used (related to ignoring results, ii) as common and critical errors.

3. Rubin, J., & Chisnell, D. (2008). Handbook of Usability Testing: How to Plan, Design, and Conduct Effective Tests (2nd ed.). Wiley Publishing.

Chapter 2, "The Many Flavors of Usability Tests," p. 23: States a common pitfall is "Testing too late in the process to be able to influence the design." (iv).

Chapter 4, "Setting Up a Usability Lab," p. 83: Discusses the value of an observation room for stakeholders but frames it as a component of an ideal setup, not a mandatory element for a valid test (supports why iii is not a core mistake).

Chapter 13, "Communicating the Results," p. 311: Discusses the importance of ensuring findings have an impact, implying that ignoring results (ii) is a failure of the testing process.