Lumped Circuit Abstraction extreme cases
Make the sources the tiniest amount imperfect, perhaps they have an ESR or 1 pico-ohm, and a voltage difference of 1 femto volt, In reality you will never have ideal sources, so parts per million errors can be mixed in while not affecting the actual results, 

Resistors on their own have thermal noise voltage, so in any circuit with any resistance there is always a voltage and current change, if you wanted to you could model any non perfect wire as an AC current source in series or voltage source in parallel if you really wanted to keep it tied to real world concepts.