Be cautious about any item you are looking at that is transcribed, copied, or printed from an original. This means there is at least one person (or more) between you and the original document. The more people, the more chances for mistakes.
If you do not have access to the original document, one option is to find at least one more printed transcription of the same document. Different printed sources may have had different people transcribing the original document. Your second transcription may also just be a reprint of the first transcription, so this type of backup is not foolproof. This is why source citations are so important in research, not just for the original document, but for the copied material. Always check for footnotes and endnotes.
Some mistakes can be small in size, but large in consequences. A transposed or miscopied date or letter can raise havoc with research, keeping you from finding a key record or piece of information. A few years ago I was checking for the name Etna in a census record in Heritage Quest. Nothing came up. Luckily, in this case I knew the name of another family member and was able to check through that. It turned out the record had been listed under the name Edna. When I later checked Ancestry, they did have the name spelled correctly as Etna. At the time I checked, Heritage Quest and Ancestry were two different companies. Different people were doing the transcribing.
Online sources can make research much easier when they provide both transcriptions and the original document, like Ancestry and Heritage Quest often do. When you are using these sources, always check for an original. I was recently following an Albany family from census to census. Suddenly, in the 1910 federal census, the wife and daughter were no longer living with the husband. At their address was only the husband and a Russian boarder. When I checked the nearbys option that Ancestry offers, the wife and daughter were living several houses away. This made no sense to me. When I pulled up the original handwritten record, there it was as bold as brass. The husband, wife, and daughter were all living together at the correct address, just as I suspected. If I had not had the online copy of the original handwritten census form to consult and had gone only by the evidence in the transcription, I would have been working with very flawed information.
Even online sources that provide both the handwritten document and a transcription are not foolproof. Remember, when you are dealing with sources like Ancestry and Heritage Quest, you are always dealing with someone’s transcription of a name to get to that original record. This is why genealogical research requires skill, patience, and mule headed determination. Luck always helps too.