The Hawthorne effect refers to a tendency in some individuals to alter their behaviour in response to their awareness of being observed.

This is why it's so often difficult to get 'true' UX feedback. The act of watching a new user (or asking them about their experience), may alter what they do.

i.e., they may spend longer reading an onboarding tooltip, if they know that they're being watched, as skipping it would feel rude or lazy.

As an example, a study found that medical staff were 55% more likely to wash their hands, if they knew that they were being monitored.

If the Hawthorne Effect can be that influential in a situation where the benefits are so unambiguous, and from a field of professionals, you can only imagine how inaccurate user testing can be.