If you buy a cheap third party battery that breaks your phone, that isn't Samsung's responsibility as far as design. That's very different to a stylus they have designed with expensive consequences from a simple mistake. There we are though. I'll never make this mistake in any case.
If the phone doesn't have a replaceable battery, there is no way a user can make that mistake since the battery can't be replaced by the user. On a phone with a replaceable battery, the user can buy a cheap battery, thinking it's from the OEM and completely ruin their phone. Because the phone allows the user to insert an improper battery, it becomes the same issue as, if the pen fits backwards, then it's a design flaw. So with that logic, phones with replaceable batteries have a design flaw because it allows the user to insert a bad battery.
Lets say people buy a cheap wall charger and cable thinking it's from the OEM, because their original one broke. It turns out to be fake and it ruins their phone. Is it the OEM's fault because they allowed fake charger to be used?
A design flaw would be that the S-Pen would have a sharp tip and it would scratch up your screen every time you wrote on it. A flaw is that if the Pen would get stuck while inserting it correctly..... If this thread was titled "The Note 6 should have a redesigned S-Pen to prevent idiots from inserting it back end first," it would have been a reasonable title and discussion. But when you view the current S-Pen on the Note 5 as a design flaw, as the thread title points out to, it's ridiculous. Insert the sim card improperly, damage the tray, the phone continues to work but the sim tray is damaged. Not a design flaw, just people not paying attention to the simple things. A pen is even easier to understand than a sim card.