The Knightly Knife's tooltip says that it deals bonus damage to armored mobs, but it does not.
Steps to reproduce:

• Start the game with WTHIT (to view mobs' health), Twilight Delight and Twilight Forest in the mods folder.
• Create a new world in creative mode.
• Obtain a Knightly Knife and a Knightmetal Pickaxe, both of which have the same "bonus damage to armored mobs" effect.
• Spawn a zombie, a mob with armor points by default, and a skeleton, a mob with the same amount of health, but no armor points.
• Attack the zombie with the Knightmetal Pickaxe, then attack the skeleton. Note that the zombie takes more damage than the skeleton because the bonus damage to armored mobs applied.
• Spawn a new zombie and skeleton.
• Attack both mobs with the Knightly Knife. Note that they take the same amount of damage, showing that the bonus damage to armored mobs did not work.
• Spawn two Snow Guardians that are wearing the same armor.
• Attack one snow guardian with the Knightmetal Pickaxe, and the other with the Knightly Knife. The knife deals more damage, so, if the bonus damage was working, it would deal more damage to the snow guardian than the pickaxe. However, it deals less damage than the pickaxe, showing once again that the bonus damage isn't working.