Not every failure takes two machines. Some are in the numbers themselves, right there in one line.
Money and floats
Computers store most decimals in binary, which can't hold 0.1 exactly, so sums drift. A float is fine for measuring distance, and wrong for money. Store money as a whole number of the smallest unit, such as pence, or use a decimal type, and decide where rounding happens.
An agent will write price * 1.2 in a float if nothing stops it, and the test passes, because the test checks one amount that happens to round well. The bug appears as a total that's a penny out on one invoice in a thousand. Say integers, say which unit, and say who gets the odd penny.
“Store money as integer minor units in a column called amount_minor, with the currency beside it. Never use a float for money, in code or in the database. Round once, at the invoice line, half-even, and test a 3-way split of 10.00 that adds up to exactly 10.00.”