Being bored of the piece is the main problem. A week before a concert or something one has to devote most time to the repertoire, but a few weeks before the piece can be in a state of experimentation. Theres so much you can do with a piece, but you have to set your self loads of limits. It's like an artist, you have to have a border, and a theme, and many other things. So you lose lots of freedom, but at the same time create the most freedom. I think that if you bear that in mind when playing, practicing, performing etc... the piece will always sound fresh. Inspiration to do something new always comes at the most unexpected time. If you play the piece too much it may sound practiced, which is boring, but at the same time you can play it so much that it sounds totally free becasue the understanding is deep.
There is always new stuff in good music. I find a lot of music slightly shallow, such as Alkan festin d'estope. That requires a lot of spontineity, and that is where the excitment comes. You could not sit for hours practicing the opening. A mozart concerto however, needs to be sooo carefully thought out. I don't believe you can ever practince Mozart to much. Theres just so much to do.