The Brown Argus (Aricia agestis) can easily be confused with the Common Blue (Polyommatus icarus).
The best way to tell the difference is to look at the underside of the wings, and more particularly the pattern of the spots.
There are two distinguishing features:
1. The Common Blue has a spot (the "cell spot") on the underside of the forewing that is absent in the Brown Argus;
2. When the butterfly is resting, the Common Blue has three dots at the top of the underside of the hindwing in a horizontal line, while with the Brown Argus the third dot closest to the head is not horizontally alligned with the other two but closer to the wing edge. We call this feature the "hockey stick".
Novel, Haute-Savoie, France, 29 July 2016