In physics, in renormalization theory, an accidental symmetry is a symmetry which is present in a renormalizable theory only because the terms which break it have too high a dimension to appear in the Lagrangian.
In the standard model, the lepton number and the baryon number are accidental symmetries, while in lattice models, rotational invariance is accidental.
References
Accidental symmetry Wikipedia(Text) CC BY-SA