Frater Thematica, Ironwrought of the Iron Hands Legion in Angel Exterminatus, who is pretty much the Professor Farnsworth of the 30k Universe. Examples of his tinkering include methods of stabilizing Medusa's 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 Fraternity for his experiments.
Vermanus Cybus: What do you have to do to be exiled from a bunch of scholars? Frater Thematica: Oh, that's easy. Both the Mechanicum and the Iron Fraternity has threatened me with expulsion many times. Dangerous experiments, radical thinking, untested weaponry. That sort of thing. Vermanus Cybus: The amount of times you've almost blown us up, I almost wish they had.
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.
Even more amusing when Malcador informs Dorn that even the Emperor thought he was joking.
In the beginning of Scars, a group of initiates are instructed to run a marathon course for as long as they can. At the end, the last initiate running, delirious out of sheer exhaustion, tries climbing up one of the Space Marines observing the exercise.
During a rare family gathering at the triumph on Ullanor, the assembled Primarchs are casually riffing on each other. After being baited by an increasingly irritating Fulgrim, the Khan shoots him down with one sentence. He's probably referring to rumours of the bio experimentation. Probably...
Fulgrim: I heard from a contact on Mars, Jaghatai, that you do strange things to your ships. Jaghtai Khan: I heard you do strange things to your warriors.\