Normally when you make a mistake in entering you passcode on most operating systems the system will lock you out for an exponentially increasing amount of time, starting at just a second but soon doubling to minutes, hours and then days.
This has been the basis of an easy prank, where you can lock some-one else out their phone for days by simply repeatedly typing in the wrong passcode.
With Windows 10 Mobile Microsoft is set to make things a bit easier for users and a bit harder for pranksters and thieves.
In Windows 10 Mobile build, when a user enters their PIN code wrong four times, the user will be prompted with a new screen asking them to enter the challenge phrase “A1B2C3.” The user is not given another attempt at the passcode until the this challenge phrase is entered, effectively putting a stop any future accidental or purposeful attempts to enter it wrong. If the user were to enter the challenge phrase correctly and proceed to incorrectly enter the wrong PIN once more, the phone will lock and require a restart.
The feature is just one more bit of polish as the operating system heads rapidly for release in what we believe is the early October time frame.
Source