The principle of equal protection requires that laws treat similarly situated entities in a similar manner unless there is a sufficient justification for the differential treatment. When a law discriminates between similar goods without adequate justification, it violates this principle by failing to apply consistent standards.
For example: A tax exemption is given only to electric cars produced by manufacturers established post-2010, although pre-2010 manufacturers offer equivalent electric vehicles. This could be contested under equal protection principles as it unduly favors one group over another without just cause.