I recently completed a autosomal DNA through ancestry. They reported that I had a possible match to another ancestry user with:
30.0 centimorgans shared across 1 DNA segment
However, it turns out that while we are related by marriage we don't appear to be related by "blood".
The only relationship we can detect is that my great grandfather is the other person's father-in-law of their great grandfather.
Assuming that our record genealogies are correct and that there was no "hanky-panky" it would seems that the suggestion of a DNA match is incorrect.
How often might this occur, and is it assignable to "experimental error" in testing? Other explanations?