Guys,We've seen several people have flashed system.img's and OTA's and ended up in a bootloop.. Not the end of the world really, BUT for some reason, before you can unlock your bootloader using fastboot, you must enable OEM unlock in Developer options in Android settings - which you cannot do if you are bootlooping.If you still have a custom recovery, you'll be fine but if you're 100%, locked bootloader and bootlooping, we haven't found a fix yet so please do not lock your bootloader.If you feel you absolutely must relock your bootloader (at your own risk) please boot the phone up to check it works properly before doing this. If you intend flashing roms and kernels or custom recoveries, locking the bootloader is not a good ideaPlease also see the below link provided by @ efrant This goes into more detail about how google have enhanced device security with 5.1 and some other pitfalls that you may wish to avoid. This is pretty salient information, so do give it a read.