The issue is not simply that some beliefs turn out to be mistaken. The deeper question is whether objective reality and the possibility of certain knowledge can be denied altogether.
One mistaken witness does not demonstrate that all testimony is worthless. Likewise, perceptual errors do not establish the universal claim that every perception must be false; that sweeping conclusion needs its own justification.
Three positions must be distinguished carefully: doubting knowledge, denying reality, and making reality dependent upon belief. They are different philosophical claims and require different replies.