Why is it Hard to "Unlearn" an Incorrect Fact?

From Scientific American:

Identifying, correcting and averting our memory errors are part of a cognitive process called memory monitoring. Incorrect associations can be tough to change, but we can use techniques to retrain our brain. When strong habits impede our ability to acquire a desired new habit or association, we experience a common phenomenon known as proactive interference. Wrong associations appear in common spelling errors such as "wierd" for "weird" and "neice" for "niece."

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