[One (M8) Q&A, Help & Troubleshooting] daniirwan:
I'm experiencing a weird problem with my M8. The unit would turn itself off all of a sudden. This happens without a low battery warning and no other error messages or warning displayed on screen. No beeps or audio cues, either. I would be looking at it and using it as per normal when it would just power down and turn off all by itself. This has happened twice in the last two weeks. In both instances, they have these in common:

a) Battery was low but not critically so. IIRC, the first time it happened the battery was at 25% and the latest incidence the battery was at 18%. The low battery warning usually given at 15% power wasn't displayed.

b) In both cases, I've just only turned the display on & started using the M8 for less than 15 seconds before it turns off.

c) In both instances, the mobile data & GPS was on. Wifi was off. Phone wasn't charging and the camera/flash was also off.

What I do after this happens is I would go home & charge the phone using the original M8 charger. The charging battery icon would be displayed and the charging level tallies with the last charge level that I see before the phone turns off. I let the phone charge for about 10 minutes or so before tinkering with it.

What's frustrating is that after these sudden shutdowns, some of my settings were reset & the phone will be wonky. I had to re-do the settings in both the settings app & also Google settings according to my preferences. Some default apps were cleared as well, so the phone would ask which app I'd like to use for my Home screen (either Sense Home or Nova Launcher, which I use exclusively). Some developer options were reset too, e.g animation scale etc. Other basic settings as well, such as auto updating apps in Google Play Store.

Also, first time it happened, I was locked out of my phone. The lock screen is displayed but all the PIN combinations I've tried seems to be wrong. The lock screen would go off after I try inputting the PIN. Restart by pressing vol button with power button didn't work at all. I'd touch the screen, the lock screen will be displayed, I'd try inputting PIN but I guess it's wrong because the screen just goes off. In the end, I had to reset the PIN number using Android Device Manager on Chrome running on my desktop PC. Only then was I able to access my phone. My wallpaper was reset, although my app icons on the desktop was the same as before. Apps were all there.

The 2nd time this happened was yesterday. I wasn't locked out of my phone but all my text messages (SMS) were lost. No other loss of data, as far as I can tell. But some of the settings, default app preferences & also app permissions were reset. Had to redo them all over again. Getting a bit tiring, this.

I don't understand what's happening. My M8 is on 6.0, stock ROM & unrooted. Never rooted. I've done the battery calibration steps once but that was before the Marshmallow update. When my M8 was running Lollipop, there were times when it would turn itself off as well but it was triggered by using the camera flash at low battery levels. However, settings, app preferences & permissions were not reset at all then.

Has this happened to anyone? What could be wrong? Would appreciate any & all feedback :) Thanks in advance.