If a person, a method, or a memory is fallible, they can make mistakes or be wrong. All humans are fallible — that is almost how the word is taught. Formal-to-neutral, and kind rather than cruel: calling someone fallible is often a way to forgive them, or to warn you not to trust a source blindly. The opposite, infallible, is the strong claim that something cannot be wrong. Though actually, people use infallible loosely (“this app is infallible”) when they only mean “very good.” Fallible sits closer to “can be wrong” than to “has already failed.”