The glitch is tough to replicate, but requires no technical know-how or tinkering. Instead, you simply press a sequence of buttons from the lock screen with careful timing, and eventually you'll be granted access to the phone in full.
I was able to replicate the glitch, bypassing a pattern lock screen on a Galaxy S3 running Android 4.1.1, while ZDNet writes that it was able to repeat the error on an S3 running Android 4.1.2. The glitch isn't easy to perform, and I succeeded only once in about 30 attempts.
Once you've successfully leapt past the lock screen, pressing the lock button subsequent times will open the phone at the homescreen every time, continually skipping the pattern lock screen. I had to restart the phone to get the lock screen to kick back in again.
Fingers crossed Samsung fixes this worrying glitch with an update sharpish, as it means anyone who physically holds your mobile could -- with enough patience -- tap their way into your smart phone, and all accompanying files therein.