Hi, lanman.
One of the first words that a junior auditor learns is "transposition". ;^}
That often occurs when the correct digits are entered, but in the wrong order; they are transposed. Like, "46" instead of "64". And it doesn't have to be the whole number; any part of the number can produce this result: $1,234.56 instead of "$1,243.56.
Transposition is the first thing that springs to an auditor's mind when the error amount is evenly divisible by 9. The answer when you divide the difference by 9 tells us how far apart the two digits are, and which column (units, tens, hundreds...) the error is in.
So your $1,800 error is most likely transposition of digits with a difference of 2 and in the hundreds column. For example: 6498
-4698 00
Other examples: 7999-9799 = -1800 (yes, it works in both positive and negative directions); 2000 - 200 = 1800; 1234.56 - 1243.56 = -9.00, which indicates a difference of 1 in the units and 10s columns. It even works when the digits are not in adjacent columns: 1234 - 1432 = -198; that's still divisible by 9 ("), but "which column" is not so obvious, except that there's obviously no point in searching the 10,000 column.
Of course, other errors, or a combination of errors, can also produce an $1800 difference, so this is not an iron-clad cinch, and it may not work in your case, but it usually makes a very sensible starting point.
RC