I agree to a point. Every tree I have seen on Ancestry containing a particular branch of my family is completely wrong. Obtaining the marriage certificate for the father and a handful of birth certificates for his children would have made the mistakes glaringly obvious, as they contain the father's middle name, which is not present in any census or baptism records and resulted in a different man with the same first and last names taking his place in those trees and his ancestral line being totally incorrect as a result.
On the other hand, I have a half-cousin who only found in later life that the woman who he believed to be his elder sister was his mother, and his parents (as he had believed) were his grandparents. But his birth certificate showed his grandparents to be his parents, as his grandmother had also been the informant and had registered him as her son to protect the daughter, unmarried and under age at the time.
So certificates are not always factually correct either. I have several where the mother has invented a fictitious father, and one where both the named parents returned to the registration office with a statutory declaration four months after the initial birth registration, as a result of which the father was removed from the certificate completely, and the mother's surname and former name were amended, both differing from the names she had originally given.
It is really a matter of not taking anything for granted, and always seeking corroboration wherever possible. In the latter case above, it was a census record that actually revealed the discrepancy which led me to the birth certificates, and an army record for the woman's husband plus electoral registers that suggest the man removed from the original certificate (a lodger in her home whilst her husband was serving overseas) was almost certainly the father of the child despite the subsequent declaration.
