After a major TF2 update, Sigmod may break, meaning missions that rely on it will not work until it is fixed. We currently disable Sigmod missions from being voted on in this case, but previously there was nothing stopping the map from loading one if one happened to be the default. This caused the map to be stuck in a state where no mission could be played and needed to be voted off to a new map.
Now, in the event this occurs, this will be caught and a different mission will be loaded instead.