Teachings or images that run contrary to the accepted dogma of the religion (or fandom, as the case may be here on Danbooru).
It is generally used in context of the Warhammer 40k franchise, where this refers to members of the Imperium of Man either going against the Imperial dogma or fighting alongside one of the many races that are against Mankind. Within the fandom, this generally refers to either a genderswap of characters with skimpy clothing, or a removal of a character's charm point (i.e. giving Ryuujou a giant rack, or straightening Tanamachi Kaoru's hair).
(Note: female Space Marines have been accepted into the 40k fandom via the Adeptas Sororitas.)