In Prospero Burns and A Thousand Sons we learn that Magnus warns the Emperor while Horus is still wounded on Davin.
In Outcast Dead Magnus hits just before the Imperium learns of Isstvan V.
Now this is obviously a glaring error, so they tried fixing it in Wolf Hunt. And they do in the stupidest possible way, by claiming that the palace psykers were holding back the psychic backlash and they only failed 2 years later, yet the Imperium still only acted once the damage occurs.
I'm sure that why this is silly and makes several character actions even more retarded has been discussed to death, but I've just read it myself and the thing that pissed me off the most is that it also negates the epilogue of Outcast Dead. The Thunder Warrior finally manages to unfuck his dying body with obvious Space Marine geneseed that they got by capturing the Luna Wolf, but now that he has to be free for Wolf Hunt the only other option is that they secretly harvested the World Eater bodies. The battle scenes at the Temple of Woe were already the sillies part of Outcast Dead, but this obvious retcon makes them even sillier.