I just assumed that a genealogist I had known for ages had correctly transcribed a date from a Virginia land record correctly. When I reviewed the record myself the date had been transcribed 10 years incorrectly. In this case, the year made a difference as it was used in part of an estimate of someone's year of death.
We can all easily make mistakes. It pays to check--your own work as well as someone else's. Sometimes mistakes are minor and sometimes they are not.