Then, of course, there wa the short story in, I think Let The Galaxy Burn. It was about a Titan detachment that found a planet on which primitive Humans rode around on massive Dinosaurs, sort of like Brachiosaurs is the impression I got. The head Princeps doesn't like this, so his Titans land on the planet (I'm not sure what types, or how many) and promptly get Dino-smashed as the beasts just shrug-off the weapons and then trample the Titans.
The scene shifts to a meeting of some Imperial Tacticians who are discussing the world. They think that organic Titans sounds rather scrummy, thank you very much, but then a thought echoes through the warp- the scene shifts again, back to the Princeps. He's tied to the forehead of one of the Dino's and is screaming 'Exterminatus' at the top of his voice again and again. The Tacticians pause for a second and then, suddenly, all change their mind about having organic Titans and sign the documant to have the world Exterminated.
Or something like that, anyway...