Tested with Skype version 3.2.0.6673 (released 1st July 2013) on various
Android devices (Sony Xperia Z, Samsung Galaxy Note 2, Huawei Premia 4G

The Skype for Android application appears to have a bug which permits the
Android inbuilt lockscreen (ie. pattern, PIN, password) to be bypassed
relatively easily, if the device is logged into Skype, and the "attacker"
is able to call the "victim" on Skype.

This can be reproduced as follows with 2 Skype accounts, and 2 separate
devices to use with Skype. The target phone is presumed to have an Android
lockscreen configured and in use, and to be locked during the test.

   1. Initiate a Skype call to the target device, which will cause it to
   wake, ring, and display a prompt on the screen to answer or reject the call
   2. Accept the call from the target device using the green answer button
   on the screen
   3. End the call from the initiating device (ie. the device used to call
   the target phone)
   4. The target device will end the call, and should display the
   lockscreen.
   5. Turn off the screen of the target device using the power key, and
   turn it on again
   6. The lockscreen will now be bypassed. It will remain bypassed until
   the device is rebooted

Similar to (ironically enough):
http://arstechnica.com/security/2013/04/crital-app-flaw-bypasses-screen-lock-on-up-to-100-million-android-phones/.
Seems that internet based calling apps might well be "unlucky".

Thanks to Emilio López for originally bringing this to my attention