There is a bug with the enchantment 'Thorns' in that having it on your armor and hitting a mob with it will cause you to take damage as well as the mob, other enchantments seem to work fine, and it is only with a 'Thorns' enchantment that this problem occurs.
MC-30602
Thorns enchantment on armor causes user to take damage when hitting a mob
Resolved
Duplicate
0
(Unassigned)
Unconfirmed
damage, enchantment, thorns
Minecraft 13w36a, Minecraft 13w36b
Retrieved
Duplicate of MC-29866, please use the search function to see if your bug has already been submitted. Currently over 57% of tickets are being resolved as duplicate.