I had an interesting time talking to some early budding genealogists today and in the course of reviewing the huge masses of information, their relatives had accumulated, I ran across an interesting phenomena. Apparently, some people can't add or subtract. In this case, the ancestral lines in question had been plowed over a hundred or more times by many would-be genealogists. But there in details of the pedigree chart was a couple from England where the husband was supposedly 23 years YOUNGER than the wife and their first child was born when the mother was 24. So the Father was 1? Further, there second child was born 25 years later when the husband was 25 and the wife was 59. All this back in the late 1700s and early 1800s.
Fortunately, many of the popular genealogy databases will alert you to these types of problems. But in a lot of cases, rather than accept the files from your relatives, you have to do the math. Certainly, the wife could have been the mother of the child born in about 1800 while she was born in 1777. But could the next child be born 25 years later. I suppose stranger things have happened but not with the same father. The marriage was supposed to be around 1800 which would support the birth of child in 1891 but not 1825 especially when the remaining children were born at regular intervals thereafter. It could be a case of choosing the wrong mother, having the wrong children in the family, the wrong husband or all of the above.
The interesting thing about this situation was not that the dates or people were undoubtedly confused, but that this had been included by a family that was so obviously involved in genealogy. Why didn't someone in the family do the math? In the early days, a large gap in the dates of the birth of children strongly suggested the death of one or more children or the death of a parent. I caught the problem because I automatically look at the sequence of the birth dates of children. When I noticed the parents dates, I suggested that he might want to investigate that whole family a little further and not simply accept the work of his relatives. It was a good entry into why we check sources and dates.
It always helps to have a liberal measure of good sense and base your research in reality. Individuals and families did not live in a vacuum. A 59 year old mother having her first child with a husband 23 years her junior is not impossible, but in the early 1800s it was extremely unlikely. Especially, as I already noted, when several children were apparently born to the couple over the next few years.
Most errors are not so obvious. You might notice a gap of only two or three years. But even then, it is a good idea to investigate further. It is not automatic that the information is incorrect, but simply questionable. This principle can be carried much further than simply calculating birth, marriage and death dates. You need to think in terms of reality at all times.