No matter how many times something is reviewed and proofread, errors can creep in. That’s a reminder to proofread your own material and to keep in mind that any published item or transcription can contain the occasional error. Some writers, editors, record clerks and tombstone cutters are more careful than others.
The flip side of this is that there are not necessarily errors in all transcriptions or errors either. Don’t assume that an original record is wrong because it disagrees with you on a key statement. It could be that you are correct.
Just because published and other materials can contain errors does not mean that they always do.