Usually when it comes to software updates on devices, official software updates that are pushed out by the manufacturers and carriers, great things are expected and customers are likely to think that they will be entering a world of improvement, no matter how little or unimportant it may seem. What they don't expect is to get locked out of their device, which seems to be the situation that some Samsung Galaxy device owners are experiencing after updating to Android 6.0 Marshmallow for the first time.
According to a Reddit thread that has recently popped up over at the r/android reddit, various users of Galaxy devices are having problems being locked out of their phones following the update, with the device failing to recognize their passwords or fingerprints that have been stored in the device prior to moving up to the new software version. So far it would appear that users have reported the issue happening on a Samsung Galaxy Note 5, Galaxy Note 4, Galaxy S6 and the Galaxy S7, which already comes with Android 6.0 marshmallow out of the box. Considering the issue on the post seems to be widespread enough to gain the reddit thread over 116 comments, it might be safer to hold off on updating your device software if there is an update available until the problem gets sorted out.
A simple work around would be to disable your password and fingerprint lock, then proceed to update your device software, however, it's also worth noting that the problem is reportedly thought to be caused by an issue with the device encryption that is messing with the stored passwords and fingerprints. It's also stated that there have been reports of this exact issue happening to users of non-Samsung devices too, so this is something to keep in mind if you have any sort of password protection set up and your device has a Marshmallow update ready. For those on Samsung devices who have already run into this problem and are locked out, you can apparently go to findmymobile.samsung.com and go through the process of locking and then unlocking the device, which is supposed to keep it unlocked, although you will need a Samsung account and have to log into it at the above listed website.