Although I cannot guarantee that all of the trees are 100% correct, I am reasonably certain of their validity. As far as possible, I have checked all data against the original source. Nowadays, however, it is rarely possible to access original source documents. I have used microfiche copies of the original registers when they were available. Otherwise I have used transcripts from reliable sources, i.e. recognised archivists, or transcriptions carried out by family history societies who normally use at least two independent transcribers. My usual source for parish records has been the library of the Society of Genealogists in London. I have also purchased fiche copies of parish registers from the Essex County Record Office.The main exception to this policy is with more recent records, where I have depended on input from people who are relying on first-hand family knowledge.
More recent trees depend on a combination of census records and birth, marriage and death indexes. BMD indexes are very useful, but unless certificate copies are actually purchased there is scope for error.
Of course, one must always consider the possibility that the source document is wrong. This can happen because of accidental error, for example a mistranscription by a clerk, or it can be the result of a deliberate falsification of the facts. As Launcelot says in The Merchant of Venice - "it is a wise father that knows his own child".