Guardians are supposed to deal you Thorns damage when you walk into them and their spikes are extended.
This no longer happens as of 14w32b.
Thassit.
MC-66044
Guardians do not give you Thorns damage when walking into them
Resolved
Works As Intended
0
Intended. Guardians only deal thorns damage if you melee them, not if you come into contact with them (unlike blazes):
http://minecraft.gamepedia.com/Guardians#Combat