Mimi Zweig, Indiana University's Professor of Violin, has preached that a non-judgmental environment is a prerequisite to success in learning an instrument (or anything for that matter!) She is adamant that mistakes should not be classified as "bad" or "good" and that all results are simply information.
The fixation on trying to obtain only "good" results can often be linked to a fixed mindset, where the student refuses to take risks and is afraid to explore different outcomes for fear of it being labeled as "bad" or not deserving of praise.
Most adults can relate to these two ideas: 1) knowing what not to do can actually be more helpful than knowing what to do, and 2) there are exceptions to every rule. This is so true in learning music effectively.
For example, intonation is often taught as an absolute, that is to say that there is only one correct way to play the note "A" and that your accuracy can be determined by measurable means, such as using a tuner. However, once you reach more advanced levels, students are introduced to at least three main systems for intonation: Pythagorean, Equal Temperament & Just, and the various scenarios where one might play a note flat or sharp on purpose, for aesthetic reasons. As such, there is no such thing as a true "A" pitch.
However, when a student is taught a fixed concept of the note "A", they tend to struggle to accept or adapt to alternatives. Anything outside of "correctness" becomes intolerable, which saps creativity and exploration. It is vitally important that "correct" results do not become the focus of a musician's efforts, but rather understanding the process.
The terms "good" and "bad" don't have a place in the practice room. At best, they make us goal-oriented instead of process-oriented, and at worst, they deny our willingness to explore and discover new things. Unfortunately, it's very easy for a student to experience what they interpret as a "poor" outcome and then to associate that with being a "poor" musician or even a "poor" human being.
We should try to focus on neutrality and the depersonalization of outcomes. Mistakes should be regarded as simply data or information which can then be assessed objectively to create actionable strategies. A thought like, "That note was out of tune" doesn't have to mean, "I'm bad at this." Instead, the thought, "That note was out of tune," should lead to, "That note was sharp by 20 cents," which then leads to, "I need to lower my finger by two millimeters." A non-judgmental environment breeds clear, objective & actionable goals.