The problem is the supplementary pair of braces, which is however necessary for correct sorting. You should try biblatex, really, because it provides different fields for sorting dates.
A possible patch is to write, after loading natbib,
\makeatletter
\let\NAT@bare@aux\NAT@bare
\def\NAT@bare#1(#2){%
\begingroup\edef\x{\endgroup
\unexpanded{\NAT@bare@aux#1}(\@firstofone#2)}\x}
\makeatother
This will swallow the offending part, while keeping the correct sorting order.
The problem is in what's presented to LaTeX and natbib to interpret. If you look at the .bbl file, you'll see that the entry starts with
\bibitem[Knuth({\noopsort{1973c}}1981)]{book-full}
Now natbib uses this to produce the results from \citep{book-full} (or \citet), by applying various commands. The relevant one (that I found with some grep search) is \NAT@bare that requires its arguments in the form
\def\NAT@bare#1(#2)#3(@)#4\@nil#5{%
The relevant part is #1(#2), which is derived from the argument to \bibitem in brackets; natbib wants to perform a numeric check, assuming that what's in parentheses is the year. But it finds {\noopsort{1973c}}1981 and this breaks off. So I make an alias to the original \NAT@bare command, calling it \NAT@bare@aux and do
\def\NAT@bare#1(#2){%
\begingroup\edef\x{\endgroup
\unexpanded{\NAT@bare@aux#1}(\@firstofone#2)}\x}
So, when natbib wants to execute \NAT@bare (it's so instructed by other macros), it will take the argument Knuth({\noopsort{1973c}1981}) and perform an\edef; thus\x` will be defined as the result of completely expanding
\endgroup\unexpanded{\NAT@bare@aux Knuth}(\@firstofone{\noopsort{1973c}}1981)
The job of \@firstofone is just to strip the braces, and the expansion of \noopsort{1973c} is just empty. So at the end LaTeX will see
\NAT@bare@aux Knuth(1981)
which is in the right format. The \endgroup balances the \begingroup and vanishes together with the definition of \x which has already done its job.
If the entry is normal, say \bibitem[Author(2012)] (by the way, Happy New Year), \x will be defined as the full expansion of
\endgroup\unexpanded{\NAT@bare@aux Author}(\@firstofone 2012)
and \@firstofone 2 will simply return 2, so in the end LaTeX will see
\NAT@bare@aux Author(2012)
as it should.
Best Answer
Since it appears that you want to have citations in sorted&compressed numeric-superscript mode, you could load the
citepackage with thesuperscriptsoption:The options
sortandcompressare "true" by default with thecitepackage.Of course, if you load the
citepackage, do not loadnatbibas well.