Association de condensateurs.
Version officielle.

Capacitors association.
The official version.

Tiré de :


My rought translation:

If one compares the energy  W' of the 2 capacitors to the energy W of the previous one, we can see that this W' energy is only half of W. It sounds like the conservation of energy principle were violated. 
Actually it is not. It has been another form of energy production which is not the same that the electric energy charge of the second capacitor. When the 2 capacitors were connected a part q''=q'/2 of the first capacitor charge was carried to the second capacitor. The wires which connected the 2 capacitors  were traveleled by an electric current  which had produced in these wires some calorific energy W''  which is, in the present case, equal to W/2= 1/4 C'U2.