After facilitating hundreds of usability tests across dozens of products, I've noticed patterns in how teams unintentionally sabotage their own research. These methodological errors lead to false confidence and missed opportunities for improvement.

The most common mistake is leading participants with directive tasks. Asking users to "Click the blue button to create a new project" tests their ability to follow instructions, not their ability to use your interface. Instead, present realistic scenarios: "You need to start working on a new client project. Show me how you'd do that."

Another prevalent error is the "friends and family" test - using colleagues or acquaintances who are already familiar with your product or eager to please you. These participants behave differently than actual users, creating a false sense of usability.

Equally problematic is over-valuing positive feedback. Most participants want to be nice, especially when the designer is present. Focus less on compliments and more on observed behavior - where users hesitate, make errors, or visibly struggle despite saying everything is "fine."

Many teams also test too late, when major changes are no longer feasible. Regular, lightweight testing throughout the design process prevents expensive rework later.

Perhaps most damaging is failure to include diverse participants. Testing only with users who match your team's demographics misses critical usability issues that affect other groups.

To improve your testing: recruit participants who genuinely match your user base; create scenario-based tasks rather than instructions; observe what people do more than what they say; test early and often; and ensure team members directly observe sessions whenever possible.

Remember: the goal isn't to validate your design, but to identify where it fails and how it can improve. The most valuable usability tests are those that challenge your assumptions.