Last week I received my seconds Windows 10 OEM pack with the product key on the COA sticker covered in silver paint, similar to a "scratchy". I can see the reason for this, but IMHO it's been badly implemented. Despite care in removing it, I managed to damage two of the product key characters underneath, leaving me with around 36x36=1296 guesses as to what they could be!
You wouldn't believe the nightmare I've been through to get this replaced. Firstly my supplier told me to call MS, and they told me to go back to my supplier, and they told be to go back to MS and then so on. After about 2x hours of my time and photos sent, I seem to have finally got a reasonable response, but it sure puts a bad taste in my mouth, considering MS have been giving free Win10 upgrades to millions over the last year, and I've bought hundreds of legal licenses for $200+ each over the last 23 years.