At the beginning of Know No Fear, Ultramarine Trooper Thiel has been marked for "censure" due to a serious, unspoken crime. Half-way through the book, after the Word Bearers have begun their devastating attack, it's revealed that his crime was... running theoretical combat scenarios on how to fight and defeat other Space Marines, mainly because he'd already run through every other possible opponent. His commanders found this borderline treasonous and had sent him to Guilliman for punishment just as the Heresy came to Calth.
Gage: [standing on the ruined bridge of Guilliman's flagship]...That was his infraction?
Jaer: Looks bloody pitiful from where we’re standing, doesn’t it?
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 The Unremembered Empire, Guilliman has found himself stuck with a permanent "escort" of Space Wolves, led by Faffnr Bludbroder, who have been ordered to stick by his side like glue*
Not to protect him, mind, but to try to do him in if they find any evidence of treason, ala. Horus and company. "If one can fall, more can fall."
. This includes when Guilliman is laying out the red carpet for his brother Lion El'Johnson, made doubly-awkward considering the long standing grudge between the Wolves and the Dark Angels. Roboute, stoic, stately Primarch of the Ultramarines, is just so annoyed Faffnr is making an already uncomfortable reunion that much worse.
Guilliman: You’re a pain in the arse, you know that?
Also in "The Unremembered Empire" Dantioch trolling Konrad Kurze about how Pollux escaped him: "The faith and will of good men,’ replied Dantioch. ‘When they stand together against infamy, the galaxy fights for them." U mad, Night Haunter?
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.\