It's not uncommon to encounter a number puzzle that has no solution unless you exploit the rules in this fashion. For example, there's a famous Henry Dudeney puzzle where you have to circle six of the following numbers to make a total of 21:
9 9 9 7 7 7 5 5 5 3 3 3 1 1 1
- It can't be done as intended, because the total will always be even. Dudeney's solution? There Ain't No Rule saying you can't turn the paper upside-down first, letting you circle three 6s and three 1s.
- A reader came up with an alternative solution; drawing a single circle around two 1s to get 11, then circling three 3s and the other 1.