Hundreds of Mennonites now living in Canada may lose their Canadian citizenship because their ancestors married in Mexico. Some 7,000 of the church members moved to Mexico and Paraguay in the 1920s, and many have returned to Canada in the years since.
Many of them married while living in Mexico, and that's what is causing the problem now. They were married by the church, and Mexico doesn't recognize church marriages as being legal.
That means their children were born out of wedlock, and they — along with their grandchildren and even great-grandchildren — are not eligible to be Canadian citizens. [Link]