Lesser Stitchwort is Stellaria graminea. The common name that I'm familiar with for most of the related flowers is Chickweed, and they all have five divided petals. Until you pointed it out, I hadn't noticed the extra petal. I checked the plant today, and all of the dozen remaining flowers have five petals. The flower that I photographed was probably knocked off by last night's heavy rain.
Wonderful macro of this delicate flower. Is six (divided petals) normal in this species? Reminds me of startworts, family Caryophyllaceae I think, but I vaguely recall they have 5 divided petals per flower. I see 10 stamens = 5 x 2?
