Leaking battery
If there’s one thing you should know about batteries, it’s that they shouldn’t leak. Ever. It doesn’t matter whether it’s in use or not. If it’s a battery, it should be dry at all times and completely sealed. If not, toss the battery in the garbage or recycle it if you live nearby a recycling plant.
When this unfortunate guy was tasked with fixing his sister’s phone, the first thing he noticed was the distinct smell of battery acid. Any sane person would’ve replaced the battery or thrown the device away a long time ago. His sister, however, was just clueless as to why her phone would overheat and turn off on occasion.