Classical antiparticles, quantum supersymmetry anomaly and constituent models

The two ways of constrained systems quantization are considered from the point of view of their self-consistency at the quantum level. With a transparent example of a particle in the external electromagnetic field we demonstrate that the procedure of gauge fixing turns out rather dangerous and may lead to a quantum anomaly in the operator algebra. We discuss additional classical symmetries as an essential element for tracing out this anomaly. The two cases of a spinning and a spinless particles in the external electromagnetic field are discussed to illustrate the situation.