Gaaah, i just put my first asphalt/bitumen shingle roof on my summer house and while i did the tiling nails properly, i messed up the roof cap - i thought wind would flap them around if i put nails under and so i put nails in corners of each cap tile, then painted them with a good layer of tar. I kind of hoped tar would cover it properly and last too.

I still have plenty of those rectangles left over, probably should just put another layer on top of it, but hide the nails properly.