While many people go into DNA testing expecting to get answers, you need to be prepared to find out information that you aren't going to like. Before DNA, when there was a non-parental event, it mostly went unnoticed in families. With DNA testing, finding out you are not biologically related to a near cousin when adoption is not involved, can be quite upsetting.
This just happened in my family. I ended up not being related to someone who should be my 2nd cousin. Where did that leave us? One of us is the result of a non-parental event. I was quite shocked, picturing my mother or my grandmother having an affair seemed mind blowing. I am my mother's oldest child and my father is his mother's oldest child. I was born 11 months after my parent's wedding but my father was born 6 months after his parent's wedding.
In the other case, the child was the last child in the family and when I checked the 1940's census, the parents were separated. How does one resolve an issue like this?
One thing that Ancestry DNA offers is the common ancestors you share with someone else you match. Through this process, I found another member of this family who I matched but my other cousin did not. I was very relieved to be able to exonerate my mother and my grandmother of having a non-parental event.
This is just one type of thing that you may find when you check your DNA. It is a lot of fun, helpful and interesting but it can also cause anxiety and unearth things that perhaps, should be best left unknown. Be forewarned and prepared for what may end up being a shock.