The main thing would be that the baking powder or soda in the mix loses effectiveness over time so you could bake it and end up with a brick instead of a cake. I wouldn't worry about it -- the low humidity in Colorado seems to add to shelf-time for things like this. Just don't bank on that cake mix working properly for a party or something -- save it for a treat for your family or to make stuff for the school/church bake sale that won't hurt if you end up throwing it out.