Taken from a well respected website which listed 15 things WD 40 is good for, like spraying on shovels etc and these things it should not be:
But Don't Spray It On:
1. Door hinges. Sure, WD-40 will stop the squeaking, but it also attracts dust and dirt. Over time, you'll end up with ugly black streaks on your hinges.
It doesn’t attract dust and dirt, dust particles will suspend in any liquid. The black streaks would be carbon from the steel, caused by friction from a lack of lubrication, the same friction that caused the squeak and then caused the owner to lubricate the hinge.
2. Bike chains. WD-40 can cause dirt and dust to stick to a chain. Use bike-specific lubricants, which typically contain Teflon.
WD-40 is great for cleaning bike chains, but given the harsh exposed environment, it is advisable to then wipe dry and apply an alternative lubricant, such as GT-85
3. Paintball guns. WD-40 can melt the seals in the guns.
Never played with a paintball gun !
4. Locks. The spray can prematurely wear down the internal mechanisms, especially in the pin tumbler locks, in door locks and padlocks. Go for graphite powder.
Whilst there are undoubtedly better lubricants, I’d be really interested to hear how WD-40 “prematurely wears the internal mechanisms” !
5. iPods and iPads. WD-40 won't repair the Home button on these devices. In fact, the spray can cause the plastic to break down on the cover, and if some gets inside the electronics, it can damage plastic parts inside.
If someone thinks it’s a good idea to spray WD-40 into delicate parts of their iPad then I think the resultant failure would be the least of their problems !