Descartes quickly found his ground floor of natural philosophy was little more than quicksand. One can almost always come up with a scenario in which unquestioned beliefs can in fact be wrong. However, it could be that our government, for reasons of its own, does not want us to know the real capital of France. They've gotten all the history and geography books, and all the maps changed to indicate that Paris is the capital, rather than what the real capital is. Someone might claim they went to Paris and saw government buildings that would reasonably be associated with that city being the capital.
People only think they went to Paris, while in reality, they never went anywhere near it.
Of course, most reasonable people would find such an outrageous conspiracy theory laughable. Perhaps because a less outlandish explanation, a less complicated solution, would seem more likely. Some might argue for mathematics. Even if one's first grade teacher were a dupe of the massive government Paris conspiracy, it seems impossible to doubt that two and two is four. One can, in fact, picture two objects joining with two other objects to form a group of four.