We keep hearing of smartphones catching fire or just getting fried internally due to the use of cheap off-market chargers (we are keeping the Note 7 mishap out of reference here because that was a manufacturing fault in the battery as Samsung recently explained). Usually, the problem with such chargers is that they use cheap or defective components that causes variations in power regulation and even outputs impure DC voltages with ripples, harmonics and even surges. All these factors affect the battery heavily and sometimes bypasses the battery protection circuit which is present in every mobile battery.
Wearables aren't left out with this problem too. Due to the small form factor of wearables such as the Gear S2 and S3, Samsung decided to let go off a wired method for charging the batteries and instead embedded wireless charging systems in them. However, wireless chargers don't come cheap and the original ones for example, the Gear S3's wireless charger, costs above $30. This encourages users to buy cheaper off-market chargers which quite often have reported to affect the wearables.
One such incident has been reported by a Gear S3 user named Lamar Jackson-
"Do not try to use wireless chargers other than OEM for your Gear. I tried a few with no luck then found one in a drawer that I had not tried. Fried my Frontier. I've been without it for two and a half weeks. Just got it back. Repaired."
It was a charge force desk mount wireless charger that Lamar had used here with his Gear S3, it seems that many other users have also faced similar issues with the Gear S3 after they used a non-OEM charger, a reddit user also reported to have burnt his wrist on wearing the Gear S3 after charging with an original Gear S2 charger. This case is more interesting as both the Gear S2 and Gear S3's wireless charger (which are certified under WPC) have similar 5V and 0.7A rating and hence outputting a power of 3.5W. This could probably be because the Gear S3 doesn't fit perfectly in the wireless charging cradle of the Gear S2 and hence causes flow of Eddy currents through other parts of the watch which builds up heat.
Another thing to notice is that most off-market Gear S3 chargers have voltage and current rating of 5V and 1A and hence outputting a power of 5W which could in some cases fry the battery charging circuit on the Gear S3's mainboard. So take care of your Gear S3 and only charge it with an OEM charger folks!