\paragraph{Infinite level ``fi''}
\def\na{\hskip 0pt plus 1fi({\tt fi})\hskip 0pt plus 1fi}
\def\nb{\hskip 0pt plus 1fil({\tt fil})\hskip 0pt plus 1fil}
\def\nc{\hskip 0pt plus 1fill({\tt fill})\hskip 0pt plus 1fill}
\def\nd{\hskip 0pt plus 1filll({\tt filll})\hskip 0pt plus 1filll}
\fbox{\hbox to 14cm{■\na ■\nb ■\nc ■\nd ■}}
\fbox{\hbox to 14cm{■\na ■\nb ■\nc ■}}
\fbox{\hbox to 14cm{■\na ■\nb ■}}
\fbox{\hbox to 14cm{■\na ■\na\na ■}}
\paragraph{65536 registers}
\leavevmode
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{\setbox40000=\hbox to 10mm{a}\copy40000
{\setbox40000=\hbox to 10mm{漢字仮名}\copy40000}
\global\setbox40000=\hbox to 10mm{asdf}}\copy40000
{\setbox40000=\hbox to 10mm{\TeX}\copy40000
{\setbox40000=\hbox to 10mm{ほげほげ}\copy40000}\copy40000}}