Caleb Johnson's MayFlowerHistory blog has noted a significant error with dates in Ancestry.com's online images and transcriptions of the original vital records of many Massachusetts towns. The error concerns the calendar change that happened in 1752. While this example points out transcription errors on Ancestry.com, I suspect the same errors have been made on many other online sites. I wouldn't blame Ancestry.com alone. Many others have made the same mistake.
If you are looking at records prior to 1752 anywhere, on Ancestry.com, on other web sites, on microfilm, or in books, you need to be aware of the importance of the calendar change made in 1752. Dates recorded before that year do not match the calendar used later, the calendar that we all assume we know.
You can read Caleb Johnson's excellent description of the problem at http://goo.gl/ueKkA.
My thanks to newsletter reader W David Samuelsen for telling me about this article.