The entire "send prawns by clacks" conversation is a Stealth Pun.Specifically, using an advanced communication system to send prawns... pr0ns... PORN.
Charlie is not only Vetinari's doppelganger but in fact the Anthropomorphic Personification of all the characteristics Vetinari left behind to become what he is now
Some of the members of the Committee to Un-Elect the Patrician are actually working for Vetinari.It's mentioned in other books that many people have tried to depose Vetinari, and that Vetinari was even behind a few of these attempts. It would be quite fitting for a master of improvisational scheming like him to keep tabs on a big conspiracy like the one Lord de Worde was running without de Worde knowing.
Mr Tulip is Borogravian.Very late in the book, it's explained that he got interested in art as a little boy while hiding from soldiers ("Ours or theirs, it didn't matter when the war had gone on this long") in a snow-bound church, which contained the only beautiful things he'd ever seen. Zlobenia is Borogravia's perennial enemy and, in Monstrous Regiment, not much mention is made of Zlobenia being ultra-Orthodox like Borogravia. Eternal war + church + cold = Borogravia.
Mr Tulip is Uberwaldian.Moist von Lipwig is Uberwaldian, and his family are members of the Plain Potato Church (as opposed to the Orthodox Potato Church).