If water condenses on your PCB, then salts left over from manufacturing processes will turn it slightly conductive. If the PCB is powered, copper from exposed pads and solder will be corroded by electrolysis. This is what kills your phone if it gets wet and you leave the battery in. It goes a lot faster than you think.
You can replace "water" with sweat, dead bug juice, rodent pee, whatever...
On a very humid day, you may hear a loud bang followed by a puff of smoke: that used to be a chineeese USB charger with 0.5mm mains creepage distance.
Now, under normal circumstances, everything has a finite lifetime...
The first thing to fail is usually a capacitor which dries up. Heat accelerates the process. If the capacitor overheats, bulges, and leaks electrolyte all over your board, this will corrode everything it touches.
Solder joints (and semiconductors) can crack under heavy vibration or due to thermal cycling (the famous XBOX ring of death).
Mechanical shock (ie, dropping the stuff on the floor) will cause heavy components to lift their pads, or just break off the board. Shock, or board flex from an over-eager user sticking a huge CPU cooler in their PC will crack brittle stuff like MLCCs or ferrites... or BGA solder balls.
Connectors which are manipulated often will wear out, or they will break off the board if they're pure SMD.
Vibration will loosen screws. Murphy's law ensures it'll fall off in the worst possible place.
Cable which is flexed too often will break.
Electrical contacts which are secured by screws can eventually loosen, then overheat and burn if high current.
Multi-strand wire will corrode over the years if the air is contaminated.
Fans fail. Air filters clog up with dust.
Software is buggy.
Semiconductors under high heat/voltage/current stress can fail due to thermal cycling or electromigration.
Eventually, flash memories will forget their contents.
Lead-free solder grows whiskers which will cause shorts.
However, unless you're designing a guidance board for a Minuteman missile, your stuff will probably be obsolete before it fails!...
What I mean is that the most likely causes of failure should be addressed first.