Look on page 174, at the bottom of paragraph 2. It says that Mulch burped due to good clay. But wait! on page 178-179, it says that because of Dwarf anatomy, it is physically impossible for a Dwarf to burp!!!
This is NOT an error! Dwarf anatomy makes sure that dwarves cannot burp while they are tunneling. Mulch was not tunneling when he burped, and therefore it is not an error.