This should not happen. Check ID's in your configs, and grep your ForgeModLoader log for the word "CONFLICT"
EDIT: Also enabling NEI's ID map dump feature is the most useful feature you'll ever enable. When you track down which ID's are conflicting, just move one of them to an unused area as indicated by the ID map. There's still potential to run into problems though, so don't trust it as the only thing right in this world, because in reality nothing is right in this world.
Thanks for the help! Upon searching the logs, I found a few other conflicts as well.