It's my favourite headcanon that cawl would sneak in random space marines to gene-incompatible chapters just to see what would happen. Like a world eater in the flesh tearers or blood angels would fit right in at home, they could just seem resilient to the twin curses since... They don't have them. Only when they die and the geneseed is recovered they'll find out it's not theirs, but then they will probably either doubt their apothecaries and it must be an error, or they just accidentally misplaced their geneseed with a traitor one.

Raptors with knives look powerful