Sometimes we see a new lug sail with a crease like this. From the throat to the clew of the sail – the back bottom corner.
It is usually because the downhaul is not tight enough – the rope that goes from the deck beside the mast vertically up to the boom and pulls it down with a lot of force.
So … lets look at a boat on the beach. Here one of the Oz Geese sailing on Lake Ta’al in the Philippines. I have borrowed the photos from the Goose website on Facebook
See the crease in the photo below? The diagnosis for a too loose downhaul is to put your hand on the boom near the front of the sail and push it firmly downward.
99 times out of a hundred the crease will disappear immediately.
You can see the same crease on on the same boat a bit later. It has reduced a bit because the sail has filled out under wind pressure. But it is still visible.
It would disappear completely with a bit more downhaul tension.
There is another thing that happens with too low downhaul tension. With some sail geometries the front end of the yard can end up on the wrong side of the mast after a gybe.
Downhaul tension reduces the chance of this happening dramatically
I’ve put together a more complete discussion on the Really Simple Sails website with some photos of a much more preferable type of crease that shows the sail is set more correctly.