Electric Potential

Electric potential is energy per unit charge, $V = kq/r$ from a point source, measured in volts, and it belongs to a location. The energy of a charge placed there is $U = qV$. Potential is a scalar, so contributions add as signed numbers with the charge signs doing the work direction does elsewhere, and geometry enters only through the distances. Dividing a potential difference by a distance gives the average field over that interval, exact everywhere only where the field is uniform, as between parallel plates. Along an equipotential no work is done, and field lines cross equipotentials at right angles pointing toward lower potential.

Five errors dominate. Using potential and potential energy interchangeably, or reporting one in the other's units. Resolving potentials into components or cancelling them by direction, when only the charge signs can cancel. Applying $E = V/d$ where the field is not uniform, or measuring the distance across the field rather than along it. Concluding that the field vanishes where the potential reads zero, or the reverse. And charging energy for moving a charge along an equipotential, or sketching field lines that cross one at a slant.