Can an employer schedule you for 40hrs and force you to take longer lunches if they think that you will have overtime?

I've read that if an employee gets overtime hours, then they are to be paid for those hours no matter what. Basically, if they have worked over 40 hours (we're talking manual labor), then they get time and a half. To be clearer, if we have 40 scheduled hours and by the beginning of our last 8-hour shift we have something like 33 logged hours, we would be asked to take an extra hour during our lunch (or leave early in this case) so that we can do a full 8hr shift without our employers having to pay us overtime. My question here is: is it legal for an employer to ask their employees (with the heavy implication that they will be fired for not complying) to take extra lunch time to neutralize any over time before the end of the pay period? Or is it just unethical?