I've recently been finding the adjective "incredulous" used in places where it was obvious that "incredible" was what was meant. While we currently think of "incredible" as a synonym of "wonderful" or "spectacular," the original meaning was "not credible," that is, "not believable," "in-" being a negating prefix, such as found in inoperable, ineffective, and inordinate.
"Incredulous" is an adjective that describes a person or a person's manner and means that the person is unable to believe something said or done.
Ex.: Tracy was incredulous that Frank whipped it out right there in the grocery store.
Physical things cannot be incredulous. Appearances cannot be incredulous. Actions cannot be incredulous.