Disclaimer: I am not a lawyer.

There are rules/laws about who can be classified as "exempt" vs "non-exempt" (salaried vs non-salaried (or the other way 'round)), e.g., whether or not you supervise anyone is one of them. If the labor board decides you should have been non-salaried, then you get the overtime you've worked...the whole time and a half. Doesn't matter if you've worked 20 hours one week, You get paid an extra 30 for the week you worked 60.