I learned this the hard way- SEEING IS NOT ALWAYS BELIEVING. As a "genealogy newbie" many years ago, I would nearly jump out of my skin with excitement when I came across tidbits of information on my ancestors. I spent hours scribbling information on family group sheets, listing new names and families, dates and descendants only to find out later that the information was wrong. Imagine my disappointment and frustration sometimes- all that hard work wasted!
I still get excited when I find new information on ancestors, but experience has taught me to proceed carefully. I look for the source that links the new discovery to my family. I examine dates and places. I look at the "whole picture" to determine if it's possible. If I have doubts, I either skip it or save the information for later reference.
I'm amazed by some of the information I find on the internet. For example, I have seen family trees where the children listed were older than the parents or the parents were over 100 years old when their children were born. If it doesn't make sense, I move on.