Check this commit for more details on the bug.
You’d be surprised to know that an operating system as mature as Android could have such a silly but quite serious lockscreen security issue. We just discovered a particular pattern lock on Android M developer preview release that can be unlocked with two different patterns.
Android M is a developer preview release so bugs/issues are to be expected, but a security issue with lockscreen is pretty serious, even for a developer preview release. Check out the video below to see it in action:
We discovered it on our Nexus 5 running Android M developer preview but we aren’t sure if it exists on Nexus 6, Nexus 9 and Nexus Player running the M preview release as well. If you’re running Android M on any of these devices, do let us know if the issue exists on your device also by setting a pattern lock as shown in the video & image above.