Electric Potential Energy

The electrostatic force is conservative, so it has a potential energy: $U = kq_1q_2/r$ for a pair of point charges, with $U$ taken as zero at infinite separation. Two things separate it from the force law. It carries one power of $r$ rather than two, and it keeps the charge signs rather than stripping them, so unlike charges have negative $U$ and are bound while like charges have positive $U$ and fly apart when released. The field's work is $W = -\Delta U$; an external agent moving the charge slowly does $W = +\Delta U = q\Delta V$. For more than two charges, the stored energy is a sum over distinct pairs, each counted exactly once.

Four errors dominate. Calling a potential an energy, so a point at $12$ V is reported as holding $12$ J regardless of what charge sits there, when $U = qV$ needs the charge. Flipping the sign in $W_{\text{field}} = -\Delta U$, or reusing the field's work as the external agent's, and concluding that a charge sped up when it slowed. Miscounting pairs, either dropping the third charge's second interaction or summing $q_iV_i$ over every charge without the factor of one-half. And carrying the absolute-value habit over from $F = k|q_1||q_2|/r^2$ into the energy, which turns every bound pair into an unbound one.