If the person one party has called as an expert doesn't know what they are talking about, that's a wonderful opportunity for the other side to discredit them. I

If the person one party has called as an expert doesn't know what they are talking about, that's a wonderful opportunity for the other side to discredit them. If your expert doesn't know what they are talking about, you should not use their testimony (see above). If nobody in the world knows whether a witness or expert knows what they are talking about or not, that's functionally equivalent to them being right, and there is nothing anybody can do about it.