Oddly placed spark gap? Samsung phone charger (ETAOU10)
It's simply there to prevent the secondary side from building up arbitrarily large static charges, nothing else.

Common-mode primary-side surges large enough to jump the gap are vanishingly rare &mdash; and if one occurs, you've got bigger problems anyway. Differential-mode surges are probably handled by other components, such as a MOV, TVS or gas discharge tube.