Doing the event quest to get eggs from the cousin's island. I found 8 after a bit of hunting but the 9th was nowhere to be found. Eventually I bought the fairy fire buff as I simply could not find that last egg. When it was highlighted I realised why I hadn't seen it - the egg was actually invisible - not 'hidden', not just behind a tree or something but actually without any art showing.
The attached image shows the egg tagged by the buff and highlighted by mouseover and it is still not visible.