Assuming we're dealing with artificial neural network (e.g. using convnets) which was trained by large dataset of human faces.
Are there any known issues or challenges where facial recognition would fail? I'm not talking about covering half of the face, but some simple common things such as wearing the glasses, hat, jewellery, having face painting or tattoo, can this successfully prevent AI from recognizing the face? If so, what are current methods dealing with such challenges?