Accessibility is often offered as an example here.
When we create a technology that requires certain abilities (the ability to use a qwerty keyboard, for instance) we have inadvertently made a technology that can't be utilized by people that lack that ability level. Likewise, for stairs, visual public signage, color use, etc.
Other examples include accessibility not based on physical difference, but social or economic difference. If you can't afford a smart phone or other computer, it can become prohibitively difficult to apply for jobs (or seek services, etc.) that only accept online applications. Or - famously - the standard IQ test used to ask what color the keys on a piano were long ago. This test was used for employment, military services, etc.; however, that question was inadvertently biased towards middle/upper class people who had experience with musical instruments.
These designs are not created with exclusion in mine, but there are often accessibility issues in one form or another because of our blind spots.
Other times, we create designs that actively mimic our biases - for instance, a bias towards rationalism led the military to design airplane cockpits back in the 1930's+ around an "ideal standard" of human physiology without any adjustability. Because no one is actually average, once you get beyond a one or two physiological measurements (maybe), these cockpits didn't really fit anyone and many US pilots died unnecessarily because they couldn't properly operate the airplane. Once the military overcame that bias they were able to make cockpits that fit many more sizes/shapes of people and our pilot deaths dropped significantly.