On my DNA results page I'm showing a close family member (1,683cM w/52 segments) and a 1st cousin (914cM w/44 segments.) The close family member share the same last name as a 2nd cousin (449 cM w/13 segments.) The 1st cousin has a different last name but has a small tree up with one person on it with the last name of the close family member and 2nd cousin. I have never heard of the last name anywhere in my tree and can't find the connection. I'm so confused. I've asked my mother and she doesn't know anyone in our family with the last name. My father and his siblings have passed so I don't know...this is so confusing. Could Ancestry made a mistake?
Errors are extremely unlikely. Many people have received unexpected results like this which were not errors. It usually means that you have uncovered a family secret.
Remember: DNA DOESN'T LIE, but it can be tricky to interpret.
If you plug the 1683 cM number at DNA Painter, you will see the possible relationships of that person to you. They are: Grandparent, Aunt / Uncle, Half Sibling, Niece / Nephew, and Grandchild. That doesn't leave a lot of wiggle room. You can probably figure out which ones fit best by your relative ages.
Many people recommend taking screenshots of the information of such close matches because sometimes they might delete their results when they see yours appear, to protect a family secret.
You're going to have to do a bit of research and detective work.
Draw up a family tree of the three close matches. Then try to figure out what your relationship to them could be that would make all the numbers and the ages work out right. Use DNA Painter for the other matches also.
Someone in your family or in their family has unexpected parentage. (Non Paternal-Event, or Not Parent Expected, (known as an NPE)). This could be due to infidelity, rape, secret adoption, babies switched at hospital, etc.
If you haven't created a tree for yourself on Ancestry, you should. Just back to your grandparents will be enough. You should look for matches that you can confirm come from your mother's or father's side of the family. By finding such matches you can confirm that part of your tree.
The only type of error that is at all likely is if you somehow submitted someone else's DNA in your test kit. One way this can happen is with bone marrow transplants. If you have received a bone marrow transplant, your results may be for the donor.