The Common Blue (Polyommatus icarus) can easily be confused with the Brown Argus (Aricia agestis).
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".

In southern European countries the Common Blue is easily confused with the Southern Common Blue (Polyommatus celina) and the Adonis Blue (Polyommatus bellargus).
The Southern Common Blue replaces the Common Blue in Southern Spain, North Africa and the Canaries.
The Common Blue, Southern Common Blue and the Adonis Blue look very similar but the Adonis Blue has a very characteristic black chequering on the white margins (fringes) of its wings, where the veins cut the margin (in Dutch: "geblokte franje").

Finally, the Common Blue can easily be confused with the very similar looking Amanda’s Blue (Polyommatus amandus) which also occurs throughout Europe. 
The main differences are the following:
1. Amanda’s Blue lacks the central basal cell spot (discoidal spot) on underside of the forewing, which the Common Blue almost always has. 
2. Amanda’s Blue is generally larger with a deeper violet-blue shade.
3. Amanda’s Blue males have a broad black shading along the front edge (costal margin) on the upperside of the wings whereas Common Blue males have a very narrow black margin.
4. In Amanda’s Blue, the white area on the underside of the hindwing between the vertical 5 black spots and the orange spots (from the top vertical black spots: between the second and third black spot) is missing.
5. In Amanda’s Blue, the orange spots on de underside occur especially on the margin of the hindwings whereas the Common Blue shows these orange spots also on the forewings. 

Macin Mountains, Tulcea County, Romania, 22 May 2018