Frater Thematica in Angel Exterminatus, who is pretty much the Professor Farnsworth of the 30k Universe. Examples of his tinkering include methods of stabilising tectonic plate movement (by causing devastating planetary earthquakes) and inventing lethal quantum rearrangement weaponry (by siphoning starship reactor energy at the most inconvenient of times). He has almost been thrown out of the Mechanicum AND the Iron Brotherhood for his experiments.
In Scars, Malcador casually remarks that he once advised the Emperor to make the Primarchs sisters rather than brothers, figuring that there would be less of the division and rivalries inherent in male siblings. Rogal Dorn is not amused.
In the beginning of Scars, an initiate after a very long, grueling test ends up trying to climb a Space Marine out sheer confusion.