Scott Pakin in comp.text.tex writes:
I suppose you haven't read my favorite example of a badly structured document: the Universal 3D File Format (a.k.a. U3D or ECMA-363) specification, which is available from http://www.3dif.org/. In that document one can actually find a section 184.108.40.206.220.127.116.11.1.3! That represents a subsubsubsubsubsubsubsubsection. I defy anyone to find a deeper section nesting in a "real" document (i.e., one not created just to demonstrate how deep section nesting can go).
I've checked this. Indeed, there are sections 18.104.22.168.22.214.171.124.1.1, 126.96.36.199.188.8.131.52.1.2 and 184.108.40.206.220.127.116.11.1.3. An amazing feat of bureaucratic writing!