It's important to note that the S-Pen can go in backwards with zero force. The pen is the same shape for its entire length, making backward insertion just as easy as forward insertion. It's something that a person could conceivably do if they weren't paying attention, and it's definitely something a small child would do.
More at Arstechnica.com
I am always skeptical of such design flaws. This looks like the same thing they said about iPhone 6 bending issues. Its not like people will actually bend their phones.
Having said that i will say i am more likely to put the stylus backward than accidentally bend my phone.