Books are enchantable with third-party mods

With third party mods, you can add enchantments (and other NBT tags) to books. This allows the player to unfairly gain advantages like sharpness X, knockback 127, etc. A video explaining this better is here: http://www.youtube.com/watch?v=gsITEKFA1RI , note that it is not my video.

This will completely wreck survival servers, allowing hackers a larger advantage.