We use data from 1222 late-type star-forming galaxies in the SDSS IV Mapping Nearby Galaxies at Apache Point Observatory survey to identify regions in which the gas-phase metallicity is anomalously low compared to expectations from the tight empirical relation between metallicity and stellar surface mass density at a given stellar mass. We find anomalously low-metallicity (ALM) gas in 10% of the star-forming spaxels and in 25% of the galaxies in the sample. The incidence rate of ALM gas increases strongly with both global and local measures of the specific star formation rate and is higher in lower mass galaxies and in the outer regions of galaxies. The incidence rate is also significantly higher in morphologically disturbed galaxies. We estimate that the lifetimes of the ALM regions are a few hundred Myr. We argue that the ALM gas has been delivered to its present location by a combination of interactions, mergers, and accretion from the halo, and that this infusion of gas stimulates star formation. Given the estimated lifetime and duty cycle of such events, we estimate that the time-averaged accretion rate of ALM gas is similar to the star formation rate in late-type galaxies over the mass range