1. Shutting down the internet.
2. Having one entity buying all Bitcoins existing at the moment and RIPing them.
3. If NP==P (http://en.wikipedia.org/wiki/P_versus_NP_problem). But this would change the whole world. Not just Bitcoin.
4. Loss of trust by the community.
5. Finding Satoshi and discovering he's the United States of America, or North Korea. Whatever makes the conspiracy more Hollywood style.