This birth certificate for James Melburn Neill from 1879 in Hancock County, Illinois, is an excellent example of why obtaining the original is frequently recommended.
This birth is indexed at FamilySearch in their database of Illinois births. When the index entry there was viewed, I originally thought that the "Melburn" in James Melburn Neill was a misreading of "Murphy" and that the transcriber had simply made a mistake. Murphy was the maiden name of Joseph's mother--Annie Murphy--and that easily could have been his middle name. I expected the original would have a difficult to read middle name that could be interpreted as "Murphy."
I was wrong.
It said "Melburn."
The second issue with the certificate is that it appears to have been completed by two different people. The letters "S" and "M" have been marked in the digital image above (which was made from the microfilm). Whether the different handwritings are significant has not yet been determined. But researchers should always consider the possibility that a black and white microfilmed image was actually a record that had been completed by two individuals.
I'm weighing looking at the original. In this case it may not be significant--the Melburn is my concern at this point. But obtaining the image told me the middle name was correct on the transcription. And the apparently multiple handwritings would never have been noticed if I had not obtained the image.
To top if off, for years I didn't research Joseph Neill. I knew that his middle initial was "M." and had concluded incorrectly that it either stood for Murphy, his mother's maiden name, or Michael, a common Irish first name.
As usual, assumptions can limit research.