Boot loop
A device that crashes during startup, reboots, and crashes in exactly the same place again — forever.
The causes are boringly predictable. An OTA update with a bug in the init code. A watchdog whose timeout is shorter than the boot takes. Or a brown-out: the radio powers up, the current spike drags the supply below the threshold, the chip resets, and round it goes. That last one looks exactly like a software bug and vanishes when you swap the charger. LG's G4 and V10 became notorious in 2016 for boot loops that turned out to be solder joints.
The defence is to let the device count its failures. A counter in noinit RAM, which survives a reset, is bumped on every start and cleared only once the system reaches a stable state. After three tries the bootloader falls back to the previous firmware or a factory image. The reset cause usually tells you why.