Back in mid-February it emerged that a hole in iOS 6.1 allowed folks to bypass the iPhone lockscreen and make calls without ever having to enter the passcode set by the device's owner. At the time, Apple said it was aware of the problem and that it was working on a fix that would arrive in a future software update.
If you're rocking iOS 6.1, you'll be pleased to know that fix has arrived. iOS 6.1.3 fixes a number of bugs, including the hole that allowed users to access certain parts of your phone without having to input the lockscreen code. The trick involved making an emergency call, canceling it, attempting to turn the phone off, and pressing the power button. Though this trick wouldn't give complete access to the phone, it did give access to the phone app, which allowed the person attempting to access your phone to make calls, edit contacts, view photos (by trying to add a photo to a contact's number), and check voicemail.
For full details on the bugs fixed by iOS 6.1.3, check out this security note (opens in new tab).