Yes! You may damage your Galaxy Note 5 S Pen’s sensor is you slide in your S Pen backwards.
This is consider manufacture defect as the S Pen are able to slot it backwards effortless. Accident does happened, you may unintentionally insert it or maybe your kids or friends who didn’t know how to slot it back the S-Pen.
I believe Samsung does aware about this flaw when it shipped the Note 5 as in the official manual for the phone, it stated clearly that the S Pen should not be placed in the device backward, lest damage occur to the phone or pen.
Check out the video below of the discovery S Pen problem when insert it backwards.
Since it has mentioned in the official manual, by damaging the sensor may not be covered under warranty. Do take note if you going to own the new Samsung Galaxy Note 5.
Source: Android Police