Some employers really don't want to pay that time-and-a-half for overtime, and they'll resort to some cheap tricks to try to get out of it. This may constitute wage theft, so make sure you watch out for things that violate your rights.
For instance, your employer may create a company policy to control overtime hours, saying that you're never allowed to work any of them without first submitting an official request and getting it approved. This is to keep workers from intentionally going over to earn extra money when the employer would prefer that they went home on time, after finding other ways to get the work done.
That policy is fine, but the problem happens when your employer comes to you and asks you to work late. Assuming you'll earn extra money, you agree. You put in the extra time happily.
However, your employer then turns around and points out that you never submitted the request or got the approval. You assumed you didn't have to since you were asked to do the work directly, but your employer says you still have the responsibility to get official approval and to inform your supervisors that the extra hours are going to push your time for the week up past 40 total hours. You didn't do it, so you can't be paid time-and-a-half.
Many employees simply accept this as the truth, which is why it's so important for you to know your rights and your legal options. The reality is that you likely need to be paid for overtime work whether or not you technically followed the policy, since that company policy doesn't overrule federal laws.
Source: Pay Scale, "3 Things You Didn’t Know About Overtime," Daniel Kalish, accessed Feb. 16, 2018