Any two pieces of conducting material, when they are brought near each other, can act as a capacitor. Often, this interelectrode capacitance is so small that it can be neglected. It rarely amounts to more than a few picofarads. In utility circuits and at AF, interelectrode capacitance is not usually significant. But it can cause problems at RF. The chances for trouble increase as the frequency increases. The most common phenomena are feedback, and/or a change in the frequency characteristics of a circuit.
Interelectrode capacitance can be minimized by keeping wire leads as short as possible, by using shielded cables, and by enclosing sensitive circuits in metal housings.