Hello,
Thanh has committed a fix to the pdftex repo (r828).
It seems that save_tail can be a node outside the current list.
I used the attached patch (pdftex_debug.diff) against r828
to print the node number, and processed following aiwg.tex.
%========
% aiwg.tex (attached)
\pdfadjustinterwordglue1 \pdfappendkern1 \knaccode\font`\f=200
\leavevmode A%
\setbox2\hbox{1f}\setbox0\hbox{2f\ }%
\unhbox2\
\end
%========
I got the following output, which indicates save_tail (4994983) is not
on the current list.
SAVE_TAIL: 4994981
node: 4994986 char_node
node: 824 0
node: 4994987 char_node A % <== from l.2
node: 4994985 char_node 1 % <== from \unhbox2
node: 4994983 char_node f % <== from \unhbox2
node: 820 11 % <== from \unhbox2
--
Hironori KITAGAWA