I would like to imagine some marines from the great crusade but pre-heresy but from a traitor-legion arrive in 40k via warp fuckery and are fucking horrified at the state of the Imperium and seeing the fanatical religious astartes, while the Imperials are led by someone only half educated on what happended in the HH and assume they are brutal traitor veterens of the seige of Terra, and are eqaully horriffied at the concept of their excistence.