Not realizing that people don't have the same level of knowledge.
<aside> âšī¸ This cognitive bias describes the significant challenge for well-informed individuals to see a problem from the perspective of those who have less information or knowledge.
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Many of us have experienced this while talking to a professional (doctor, lawyer, college professor, etc.) or a skilled tradesman (plumber, draftsman, electrician, etc.) that are speaking from an area of knowledge that we don't share or understand- if they don't make an effort to "dumb it down" we have difficulty understanding what they are saying .
For example a doctor can use a lot of long technical words to describe straightforward ailments such as a broken arm or leg. These long words will contain information about the name of the bone, the location, the severity of the break, the type of fracture, etc. (all things that are necessary for documentation and treatment for the doctor) but more than the average layman needs to know.
For example, if there's only one decision-maker in the product team, and he is overconfident in his knowledge, then with a high probability, he will be prone to the curse of knowledge. This will lead to a situation where other team members will try to maintain the status quo while the decision-maker will think everyone shares his thoughts.
Or, let's take another example. If the product team shares the same expertise and is generally on the same page, they can collectively decide to skip user testing for some crucial features with a single argument like "Our users will definitely love this feature!"
Obviously, such an approach might be disastrous, regardless of how much belief and confidence the team puts on the table.