The Berlin Wall was reality. It was "true" for anyone alive at the time.
There are many other examples in life where things that had once been accepted as true fell under the weight of a new reality: The Earth is flat. Man can't fly. Smoking doesn't cause cancer.
Some of these beliefs go back to things said to us or about us when we were kids ("you're just not good with Math") or to what we picked up from our parents or other authority figures (coaches, teachers, etc).
False or limiting beliefs include things like, "I can't save any money," or "I'll always be fat," or "I can't cook." They hold us back. They stop us from trying new things, branching out, taking risks.
The challenge is to recognize the false belief when it appears and to ask yourself if it's really true or merely something you just accepted as true.
Ask yourself, "Who says?" and embrace a new reality.