Some smartphones require a password every time an app is purchased. Others don’t – or permit a one-click purchase in a short window after the system password is entered. In any case, it’s easy to accidentally buy an app yourself – or for a curious youngster to do serious damage to your wallet.

You have some recourse here, but nothing is guaranteed. If you use an iPhone, you can appeal your purchase. Find the necessary form by following a link in the electronic receipt iTunes sends to your e-mail. Just click Report a Problem at the bottom of the receipt. You’ll be taken to a website with a complaint form and a drop-down menu. Select “I inadvertently purchased this application” from the menu and describe the circumstances in the comment box. There’s no certainty that you’ll get your money back, but Apple is known to be lenient toward those who go to the trouble of asking.

Android app refunds are either much easier or far trickier, depending on your situation. If you notice your mistake immediately – for example, your young child taps Buy just before you’re able to snatch your phone from him – you can uninstall the app for a full refund. The catch? You’ve got 15 minutes to do it. (In the Android market, select Menu and then My Apps, tap on the app, and click Uninstall and Refund.)

If the 15-minute window has closed, you can sign in to your Google Checkout account from a computer and request an order cancellation using the online form. In this case, though, you’re at the mercy of the app’s seller. Good luck.