Retrofitting of natural gas-powered premix burners to accommodate hydrogen is limited by the flashback propensity of hydrogen-enriched flames. In this study, flashback initiation is investigated for hydrogen-enriched premixed laminar methane flames. A domestic gas boiler burner is adapted to enable high-speed intensified imaging of its internal part during transition to flashback. A zoom on the location of flashback initiation gives clear evidence for the existence of two distinct regimes. In the first one, the initiation of flashback is due to the upstream propagation of a flame, initially stabilized on the outer surface of the burner wall, through a burner hole. In the second one, spontaneous autoignition of the reactants occurs inside the burner due to their preheating at the contact of the hot internal burner wall, initiating the flashback. Experiments conducted over a large range of hydrogen-enrichment and equivalence ratio confirm these observations. An autoignition Damkohler number Dai is defined, providing theoretical support to the distinction between the two regimes. These conclusions are finally confirmed in a canonical configuration. They may have significant consequences on the conception of hydrogen-enriched premixed laminar burners.