How come the Halo 3 legendary Edition Bonus discs say insert game disc first?

[Anthony N]

Theres a sticker on the front of the bonus disc case that says "IMPORTANT! Insert Game Disc First." Why does it say this? Why do you have to insert the game disc first, before the bonus disc and what will happen if you don't?

 

[cheda50]

It's because the bonus disc has content that reveals the ending of the game and it could ruin it for you