  We address the Klein-Gordon equation for a spinless charged particle in the
presence of an electromagnetic (EM) field, and focus on its known shortcoming,
related to the existence of solutions with a negative probability density. We
disclose a principal way to overcome this shortcoming, using our recent results
obtained in the analysis of quantum phase effects for charges and dipoles,
which prove the need to abandon the customary definition of the momentum
operator for a charged particle in an EM field through its canonical momentum,
and to adopt the more general definition of this operator through the sum of
mechanical and electromagnetic momenta for the charged particle in an EM field.
We show that the ap-plication of the new energy-momentum operator to the
Klein-Gordon equation actually eliminates solutions with negative probability
density. Some implications of the obtained results are discussed
