If it sounds too good to be true, it probably is.

The Better Business Bureau has made note of many different versions of the same trick. You’ve likely seen some of those online ads offering “free” trials, subscriptions, samples, and so forth, but you won’t find out that they’re not free until too late.

Companies in many industries offer legitimate free trials so you can test the waters before deciding to purchase a pricey subscription. They can be a great way to try a new product or service, but you should be aware that in some cases you might be agreeing to purchase additional products or services if you don’t cancel your free trial.

The idea is that you become enticed by a free product or service, so you sign up for a free trial. When you sign up for the trial, you give away your credit card information and don’t realize that your “trial” won’t be cancelled, but rather turn into a paid subscription. At this point, the company can extract payments directly from your credit card on a regular basis. You may not have been told that you’ll have to start paying should you not manually cancel your “free subscription,” but the company will keep billing you. Once you’re in a situation like this, it’s difficult to cancel your subscription without paying a hefty cancellation fee.

To avoid having this happen to you, pay close attention to the “material” terms that the ad uses. By law, companies must disclose the material terms of a trial before they can accept your consent. The material term that matters in these cases is the length of the trial. Many trials now automatically transform into paid subscriptions once the trial period runs out, and it’s now the consumer’s responsibility to cancel the trial before the paid subscription kicks in.