An action that takes place that could change the locale or even the world has got to be a big incident. You'll sometimes see this on the ends of "incident" names, like the "Manshuu Jihen" (満州事変), aka the "Manshuu Incident." You know that it isn't an unusual or strange incident (like 変事) because the "strange" kanji (変) doesn't come first with this word. Instead, it comes after and means "change" in this case.