The propensity of hydrogen-enriched flames to flashback poses a challenge when retrofitting natural gas-powered premix burners to accommodate hydrogen. This study investigates how the burner geometry influences the mode of flashback observed in laminar premixed methane/hydrogen-air flames, which are stabilized above slits. Experiments are conducted on a series of 0.6 mm-thick multi-perforated plates with a fixed slit length of 6 mm and widths ranging from 1.0 mm to 0.3 mm. A transparent injection setup facilitates high-speed intensified imaging of the flashback occurrence. Two distinct initiation regimes are identified. The first one designated as 'hydrodynamic' involves upstream flame propagation through a slit, while the second, termed 'wall-ignition', entails autoignition of reactants in contact with the hottest spots of the inner burner wall. Both wall temperature and autoignition delay times are found to be insufficient for distinguishing between the flashback regimes. Instead, an autoignition Damköhler number Dai comparing a gas residence time to the autoignition delay is used to distinguish the flashback regimes. For Dai > 2, only wall-ignition is observed. For Dai < 1, only hydrodynamic flashback occurs. An intermediate region 1 < Dai < 2 where both flashback mechanisms can be triggered is revealed. Finally, reducing the slit width is shown to be ineffective in preventing flashback. Instead, the coupled heat transfer between the flame, burner plate, and fresh gases increases the propensity for wall ignition as the slit width decreases. These findings prompt a reconsideration of the design of hydrogen-enriched premixed laminar burners.