OOPS! Schengen overstay, [closed]

Several visits to Schengen in 2017. Due to arithmetic error, totals more than 90 days. U.S. citizen.

Don't want to brag about it—"You didn't catch me, needed,neener,neener!"

But I'd like to have with me on the next visit something(s) to pull out in case I get confronted with it. What sort of thing should I prepare? Should I add the amount of overstay to the ninety days of absence? Or to the 180 days from first entrance?

Or to put it another way, what could a person obtain/prepare/present to avoid problems after an unintentional overstay?