These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
There is one against Matt Ward - his Creator's Pet, Kaldor Draigo, has become the favored Angyl Prince of Star Father, while other Grey Knights considers Star Father to be perversion of what God Emperor stood for. In addition, you know how they say that Kaldor Draigo exists because GW didn't let Matt Ward bring Roboute Guilliman back to life? Here Guilliman returned and banished Draigo back to the warp. Then died.
There is also a second one, also against Matt Ward and in this case his Codex: Gray Knights, which included a Dreadknight, a sort of small, powerful mecha in which driver had exposed head. Here Brother-Captain Stern riding one is killed by a single headshot from lasgun, perhaps the weakest military-grade weapon in setting, as he is about to kill Abbadon.
A third briefly mentioned one exists in the Uncle/Emperor's activities. One paragraph notes he rescued a large detachment of Sisters of Battle from the Bloodtide as it took over the Far-Veil system. Anyone who has read the Grey Knights codex will know the exact incident this is mocking.
A fourth is in the war between the Tau Remnant and their allies and the Chaos realm of the Terran Hells. The allies are convinced at first that their technological advantage will mean the demons will be no match for them, as the forces of the Terran Hells are armed merely with spears and bows and other Iron Age weaponry. They are wrong in that assumption. The name of this blood-soaked conflict, given that it's being fought over to rescue AunīVa from being corrupted? The Salvation War.