Quote:"Clerical or Mechanical Errors. A clerical or mechanical error occurs when a Coin is encapsulated with a label that bears a grade and/or description that clearly does not correspond with the Coin. It is the duty of the buyer and seller of a Coin to examine such Coin for a clerical or mechanical error to return such Coins for correction when warranted. This Guarantee does not apply when Guarantor determines, in its sole reasonable discretion, that a clerical or mechanical error has resulted in the Coin having an incorrect grade or description. If requested, any clerical or mechanical errors will be remedied free of charge by updating the encapsulation to show an appropriate label.
NGC certification labels with incorrect dates, mintmarks, denominations or Coin types (all of which should be obvious to someone who performs an inspection of the Coin and label) are considered clerical or mechanical errors. Examples of such errors include, but are not limited to, the following:
A Coin is labeled a 1955 Doubled Die
Lincoln Cent, when the Coin has no doubling.
A Coin label description references a 1916
Standing Liberty quarter, when the Coin is a 1916
Barber quarter.
A Coin is identified as a Proof, when it is a business strike and the types are readily distinguishable from each other.
A Coin is described as an 1881-O
Morgan dollar, when it is an 1881-S
Morgan dollar and the mintmark was improperly identified on the label.
A Coin is graded MS 68 when it should have been graded AU 58, an error with respect to grade that would be obvious to a collector."
Although if someone were to buy the coin sight unseen I think that may be a problem.
I think the main distinction is if the error is obvious. It's pretty easy to distinguish between an AU-58 and MS-68 coin, but if it were like MS-65 VS MS-68, It may fall under the guarantee even though it's possible it was a mechanical or clerical error.
Clerical errors aren't particularly uncommon though.