I do a lot of genealogy and often see people posting on forums that they just got their DNA tests back and are shocked to see xxxx ethnicity. But, the results are only rough guesses that are only as good as their testing pool. (If you test for health genetics at 23&Me, the results should be accurate though.)

I tested at Ancestry and uploaded my results to FamilyTree DNA and GEDMatch.

• Ancestry says that I am about 90% Irish, Scottish, English and German (they break it down more), about 4% Scandinavian, and the rest trace stuff. This matches with the paper genealogy work I've done and I have DNA cousin matches to all these branches of the family.
• FamilyTree DNA says that I'm 89% British Isles and 6% Iberian (Spanish and Portuguese). No German. However, some of my most recent immigrant ancestors were from Germany. Also, I am descended from Mennonites who were almost exclusively German. As I said, I DNA match to cousins in these branches, so the lack of German in the FTNDA results cannot be correct. But if I hadn't seen the Ancestry results, I might be shocked by FTDNA.
• GEDMatch has nearly a dozen different testing pools I can compare against and the results are all different. One says I'm nearly 14% Russian and more than 11% Mediterranean. I expect to see some traces of non-Northern European mixed in--after all the Ottoman Empire controlled much of Europe until after my ancestors started immigrating to the US--but those numbers are abnormally high.

People also often get quite excited about small, unexpected results: "I'm 1.6% Native American!" But anything under about 2% is just noise. Autosomal DNA just isn't that accurate for ethnicity results at that distance.

If you're strictly from an endogamous community, you probably already know this and DNA isn't going to tell you much more. The biggest group this affects is Ashkenazi Jews. Your results are going to say you're Ashkenazi Jewish from Germany, Central Europe, Poland, Russia, etc. and everybody has the same results. The tests just can't parse the results better for communities where people are strict about marrying within the community.

Claiming native ancestry was really common with white people for a long time, too, because they didn't want to admit they might have African ancestry. Part of my family is from the South. My DNA results tell me that I don't have African ancestry. But matches to distant cousins on Ancestry who are African American tells me I have an ancestor who's behavior I should be embarrassed by. I don't know who he was yet, but I'm sorry someone I'm related to was such an a-hole and assaulted women in that way.