I have a commercial module addon that gives my designers access to their products, sales and coupons etc through a seperate folder so they don't access the main admin folder and when adding a coupon it is automatically set to allow their master category so the coupon is restricted and not valid on the whole store lol

Many times we gives out coupons that we only want to allow the customers to use on a single product, which is where both those allow restrictions come in.

I could be wrong, but I believe this error is only happening when the coupon is restricted to a single use (not just per customer, but actual one-time use), I have yet to see it happen if the usage is unrestricted or restricted to 2 or more (cross fingers lol) and as soon as I remove the top allow for the category the error doesn't happen any more.

I have all security patches and bugfixes applied....as far as I am aware I didn't miss any, but it could happen LOL