Example would be that Wikipedia doesn't consider the Bible to be a reliable source. In some cases, a person will come to Wikipedia to change some part of an article about themselves only to be told that they are not a reliable source about themselves!
This sound ludicrous on its face. How in the world is a person not the foremost expert on themselves you ask? Let's go over Wikipedia's guideline on Reliable Sources and come back to this question.
Wikipedia's guideline on this topic is simply called, "Identifying reliable sources" and the fact that it's a guideline and not a policy means that it's sort of a grey are. There is not bright line between what is a reliable source and what isn't.
Wikipedia summarizes this guideline in this way: "This guideline discusses how to identify reliable sources. The policy on sourcing is Wikipedia:Verifiability."
Wikipedia's guideline called Verifiability is summarized in this way: "Readers must be able to check that Wikipedia articles are not just made up."
The Verifiability guideline states that any questionable information (which is almost all information) must be able to be verified while the Reliable Sources guideline outlines how to determine if a source can be trusted or not so that information have be verified.
Put those two together and you have a cornerstone of Wikipedia that directs users on how to make sure that content on Wikipedia is accurate. Have information that's arguable? It needs to be verified! How do we verify it? With a reliable source!
Hours could be spent discussing where the line is between reliable and unreliable but when it comes down to it, if the reliability of a source is questioned, the editors involved do their best to come to a mutual agreement. Sources like The Washington Post and The New York Times are generally accepted to be reliable while a small blog written by an anonymous person who uses anonymous sources is not reliable.
So back to the question asked: How is a person not the foremost expert on themselves?
The simple answer is that sometimes people forget and sometimes people lie. If you asked someone who committed a crime what the "truth" is, they may say one thing while the police, using evidence, say another. Who is reliable in this case? Wikipedia's answer would be to cover both sides.
What is someone contests the date of birth found on their article? If there's a reliable source that disagrees with the person in the article, who do you believe? The person who the article is about, obviously! The problem is that anyone can claim to anyone else on Wikipedia. Today, you can create an account on Wikipedia called TomHank99 and claim to be Tom Hanks. It's been done but what that editor is not considered a reliable source until their identity is verified.
Verification and identifying reliable sources is very important to Wikipedia's editors. If they're not sure, they'll assume that the source isn't reliable, especially when it comes to article about living people.
Articles about living people or Biographies of Living Persons have a higher set of rules to follow but we'll cover that in a later post.
So remember, Wikipedia aims to be accurate and that requires verifying information given by reliable sources. Sometimes the results of that aim can look weird to the outside world but there are guidelines in place to make sure that the information you read on Wikipedia is as accurate as it can be.