Unusual arcing or sparking inside a microwave or speed-oven cavity is not a minor cosmetic issue — it points to metal contact or interior damage that needs a professional inspection before the unit runs again. Continuing to use a unit that’s arcing risks damaging the magnetron further or creating a fire hazard.
Unplug the unit (or shut it off at the breaker) and book a diagnostic visit rather than running another cycle to see if it happens again.