I am no expert, but I used to work for a surveyor friend on my days off from my regular job.
Land surveys are a matter of historical data and all surveys must be filed at the county courthouse in the locale where they exist.
When a surveyor begins a new survey he (or she) must start from a known point, ie: an existing survey marker or monument.
Some of the older surveys we researched as starting points were actually worded as such: "seventeen paces North of the large boulder at the corner of -______. (As an example)
Those survey markers/points ARE what the law must go buy if that is the registered historical data point.
Many, many fences are not on property lines due to uneven terrain, waterways, or even inethical landowners.
Onx obtains their property lines and corners from the local county courthouse's written historical records, thus if there is a mistake in identifying a corner properly it would either fall on Onx or the county recorder as being negligent in accurate data entry or preservation. In wither case one would hope they have proper protections or insurance in such cases.
This is just from my 10,000' view and I am not a land lawyer or legal expert... but I did sleep at a Holiday Inn once.