All of us probably have run across such "facts" in our own genealogy data from time to time. Perhaps it was a product of our own poor typing, or perhaps it snuck into our file when we quickly downloaded a GEDCOM and merged a record or two without carefully checking the information (OK, maybe YOU have never done that, but I admit that on occasion I have been guilty!).
A recent article on Dick Eastman's Online Genealogy Newsletter titled "A Sanity Check for Your Genealogy Data" addresses this topic. The article also explores how to use your own genealogy program to help identify the problems when they exist.
You can read the full article at: http://blog.eogn.com/eastmans_online_genealogy/2009/04/a-sanity-check-for-your-genealogy-data.html