During Season 2 there's a scene where Izie (I think) is explaining how the last Digidestined gave up their crests to restore the world. However, I don't ever remember seeing that. The only time I remember seeing them loose their crests was when the Tags and Crests were destroyed in the last episode of the first season, and they realize that the real Crests had always been inside them.

I'm wondering if there was an OVA or something that takes place between the first and second seasons where the digital world is brought to ruin and the Digidestined have to give up their crests to restore it.