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Nothing will bring gameplay to a standstill like the spectacle of watching the enemy fail roll after roll, blowing themselves up with misthrown grenades, misfired guns, and their own accidentally primed belts of grenades. Not having to fire a single shot, or even move from the starting positions, even (in the Rogue Trader's case) remain completely unaware of any combat.
The Deffwotch campaign: A festival of Orks being orky and "sneaky", and people failing to notice they're Orks (and those clever enough to notice also being clever enough to hide it).
The rulebook can get very snarky about critical damage. One of the possible outcomes involves a person's face being horribly disfigured and being permanently blinded. One of the side effects is that a d10 must be rolled to determine the character's new fellowship unless said character already had a fellowship of 10 or lower. Because, to quote the book "Nobody really notices the difference."