It seems that Samsung’s update to Marshmallow for a few of the higher-end models have been causing users some grief. Although it doesn’t seem like a common issue, some people have been locked out of their device entirely.
After the Marshmallow update has finished installing, some people found that neither their fingerprint nor their password worked to unlock the device. Those with active Samsung accounts could unlock them online, but otherwise they were stuck.
The cause of this issue is unknown, though encryption being enabled after the install is suspected. Either way, there’s a small risk of locking yourself out if you’re updating to Marshmallow. Turn off security before the update and you should be fine. Hopefully Samsung will soon fix this issue.