Why trials turn into charges
A free trial that asks for a card is usually a subscription with a delayed first payment. When the trial ends, it renews into the paid plan unless you cancel first. Nothing new needs to be agreed, because you agreed at the start.
The trick is not hidden. It is just timing: you decide on day one and pay on day eight, and nothing in between asks you to think about it again.
Cancel where you signed up
Cancel with whoever is billing you. If you started the trial inside an iPhone app, it is in your Apple account's Subscriptions list. On Android it is under Subscriptions in Google Play. If you paid with PayPal, check its automatic payments. Otherwise it is in your account settings on the company's own website.
Deleting the app, unsubscribing from the emails or closing the account page does not cancel a trial. Only the cancel button with the billing party does.
Cancel early, not on the last day
Many services let you cancel right away and keep using the trial until it ends, so there is often no reason to wait. Read the cancel screen before you confirm: some services end access immediately, and that is worth knowing first.
If you want to keep trying the product, set a reminder for the day before the trial ends at the moment you sign up, not later. Later is how trials become charges.
Check that it actually stopped
Look for a cancellation email and keep it. Then check the subscription list again: a cancelled trial usually shows an end date instead of a renewal date.
On your next card statement, look for the merchant name. If a charge appears after you cancelled, start with the company or app store's support, and save the cancellation email as proof. A trial that rolled over and was then forgotten is what people call a gray charge.
The subscription with no trial
Forget Your Credit Card has no free trial and nothing to sneak past you. You pick a monthly amount from $1 to $1,000, Stripe bills it, and confirmed payments count toward a public Lifetime board if you choose to appear.
STOP FORGETTING cancels at the end of the current paid period. Nobody wins anything. The only surprise is that someone chose it on purpose.