An important component in using the internet for genealogy research is in learning what sorts of information are proof, and which bits of data are merely clues. There are so many researchers posting information to web sites, query boards, and personal family sites that by sheer volume, it stands to reason that there will be errors in some of the data. We all make mistakes as we type, so the first clue in online genealogy research is to only trust original sources. Any transcribed information inherently carries with it the possibility of errors.
Of course, we all know that there can also be errors in original documents, but those are not the kinds of errors that can happen when a help researcher types in a wrong birth year, place or other bit of data.
Some well-meaning family historians will also gather a collection of data about an individual or family and then begin to feel that they "know" them well enough to make assumptions about things. A common example of incorrect assumption is when a researcher learns that a family moved west in 1890 by wagon train. In today's society we seldom see families separate so we assume that every member of the family moved at the same time. However, it was no uncommon for one or two members of a family to move west, establish residency, and then send for or go back for the rest of the family once they had things settled.
Some researchers protect themselves from making assumptions by not adding any information to their file or genealogy program data unless it is proven by at least one source that can be documented. Others keep "possibilities" in a separate folder or section of the genealogy program and label all items in this section as "supposition" or "undocumented."
Documentation and trustworthy sources are the hallmark of a conscientious family historian. Be diligent to document where you found all information. Be as accurate as possible when entering information into your genealogy program, and only enter information into the "proven" section of your data if you have a source that can be documented. View information from other researchers as "clues" rather than "proof," and the information in your genealogy files will be reliable.