%%% Why \overfull

The following code gives overfull boxes. Why?

\beginverbatim
\tracingparagraphs=1
\nopagenumbers
\parindent=0pt
\setbox0=\hbox{C}
\hsize=\wd0

C\par

\discretionary{AA}{BB}{C}\par
\endverbatim

% does not apply to luatex!