= Solution
A <quantum anomaly> occurs when a symmetry of the classical action cannot be preserved by the regulator or <functional measure> of the quantum theory. An anomaly in a global symmetry is physically allowed: it changes a classical <current conservation> law and can mediate observable processes. A <gauge anomaly> is fatal unless it cancels, because gauge symmetry is the redundancy that removes negative-norm and longitudinal unphysical states; its loss spoils the gauge Ward identities and unitarity.
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