#NOTE - THIS EFFECTS SURVIVAL AND CREATIVE#
Protection and attacking based enchanted books, when combined cause the enchantment in the second slot to be erased.
1. Combine Sharpness III book with Bane of Arthropods III
2. The resulting enchanted book will have the enchantment from the first slot, Sharpness III, effectively erasing the Bane of Arthropods enchant.
I expected this combination to not work.
Attacking based enchants - Sharpness, Smite, Bane of Arthropods
Protection based enchants - Protection, Fire, Blast, and Projectile Protection
Mining based enchants - Silk Touch, Fortune
This bug will work with the above enchants having different levels also.
See screenshots for examples.