Capacitors in parallel with voltage sources
The problem is that you can not connect an ideal voltage source of a given voltage in parallel with an ideal capacitor that has some initial voltage different from the source voltage. Once these two are connected, our definitions of "ideal voltage source" and "in parallel" demand that the voltage across the capacitor instantaneously changes.

Now, since $$i_C = C\frac{dV}{dt}$$ we have a finite change of voltage in zero time, which requires an undefined/infinite amount of current. Therefore, such a connection is not valid within our definitions of ideal circuits.

In the real world, all wires are also resistors and there are no ideal voltage sources, so what you see is a very large current flowing through the capacitor until it reaches the voltage of the source.