This paper investigates the possibility of autoignition as a flashback initiation mechanism in laminar premixed burners powered by an increasing content of hydrogen in the fuel blend. A specific experimental setup provides an optical access inside a generic burner reproducing the main features of domestic boilers. This window is used to gather high-speed intensified images during transition to flashback. Zooms into the location of flashback initiation provide clear evidence for the existence of two distinct regimes. In the first one, flashback is initiated by a flame stabilized above the burner that is able to propagate upstream through one of the burner holes. In the second regime, flashback is initiated by autoignition of the fresh gases inside the burner, as they flow along the hot metallic internal wall. These observations are corroborated over a large range of equivalence ratio and hydrogen content. They demonstrate the dependence of the initiation regime on the burner wall temperature and highlight the role of the crossover temperature of hydrogen-enriched mixtures in triggering autoignition. An autoignition Damkohler number Dai that compares a residence time of gases along the hot wall and the autoignition delay time is defined. The ratio Dai is found to be more than an order of magnitude higher for autoignition-induced cases compared to the cases initiated by a flame propagating upstream, further supporting the existence of two distinct mechanisms leading to flashback. These results carry substantial implications on the design process of H2-enriched laminar premixed burners.