* 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 ''[[{{Squick}} 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 [[http://1d4chan.org/wiki/Deffwotch 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).