If you do forget, that month's charge will remain as a credit on your account.
You can see the number of credits you have available under your App's settings menu.
You can always 'PUBLISH' that credit next month. Each subscription payment (monthly or quarterly) acts as a credit. That credit can be used for up to 12 months, as long as you maintain an active account.
The app will allow you to publish those credits until they are used up, and it will take them into consideration on checkout.