Back in August during the launch of the Galaxy Note 5, an issue was discovered that could potentially break the device. If you stuck the S-Pen in backwards—pointy part first instead of flat part first—it would get stuck. From there, you'd have to forcefully remove the S-Pen, which could potentially break the top off the pen or damage the pen eject functionality.
Samsung's first response to the problem was to ship the Note 5 with a warning sticker on the screen, but now Phandroid has discovered that new Note 5s will offer a real, physical solution.
Pen detection on the Galaxy Note 5 was handled by an internal switch that the pen would hold down on when it was inserted. When put in backward, a void in the top of the pen would catch the spring-loaded switch, trapping the pen. Samsung has now put a flexible, plastic cover over the switch, which acts like a ramp. When the thinner part of the S-Pen passes over the switch in this setup, the ramped plastic allows the pen to press the switch down rather than catch it. (The cover also seems to be attached with adhesive tape.)
Samsung hasn't said it will be offering this fix to existing customers. So for most owners, the advice is still "don't do that."