I guess not knowing what paint to use on softer plastics springs to mind. Mattel's Secret Wars Daredevil with enamel black paint detail that never dry and rub off even it the unopened package. I remember an early Toy Biz Namor who's black hair remained tacky. Who doesn't like a little extra poison with their toy?
not to forget the overly glued and often overly painted McFarlane toys of the 90s. The defence, 'These toys are for collectors, not children.' didn't hold past the fact that many were broken sealed in package, and the complaints weren't coming from parents of sad children, they were coming from adult collectors.