Well, the Warp in the WH40K universe is a pretty unpredictable method of travel. All sorts of weirdness is to be expected.
Navigation Officer: "Sir! Warp jump completed. Uh, we've only traveled two hundred meters forward."
Captain: "By the Emperor's testicles..."
Navigation Officer: "And... two hundred years back in time."
Don't start anything you can't finish. Preferably to component atoms.