The new Mac App store offers convenience and new ways to discover applications for OS X, and though Apple has tried to make the store's experience as simple as possible, in its simplicity, a crucial step has been overlooked that may result in inadvertent purchases of applications.
While you can browse through the store, in order to download items (even free software), the store requires you to enter payment information in the store's settings. This information is requested of users immediately when signing in if it is not already linked to your Apple ID.
Doing this immediately turns on one-click purchasing behavior, which is the only purchasing option available in the store, and as a result, a few people have experienced problems where inadvertent clicking of the Buy button (which can be done if you are clicking the menu next to it for additional linking and sharing options) results in an immediate charge to their credit cards. Even if the download is canceled before it completes, the charge will still go through.
While you should be able to contact Apple at the Mac App Store Support page (click the Account and Billing section) and dispute the charges, this may be a bit of a burden, and in the meantime your credit card will be charged.
In addition to inadvertent clicks of the Buy button, some initial bugs with the store's interface may result in inadvertent purchases. One user on the Apple Discussion Boards mentions having clicked the icon of an application to get information on it, but instead of showing the desired information, the store registered it as a purchase and began downloading the application.
Hopefully Apple will provide more purchasing options and confirmation steps in future versions of the App store, but for now workarounds for avoiding this behavior include staying signed out of the App Store using the "Store..." menu or removing credit card information from the store's settings until you are ready to make a purchase.