Looking for an (old?) problem with \\

You have good memory, the change predates LaTeX2e but in the latex sources you find...

  \begin{macro}{\@gnewline}
 \changes{v1.2u}{1996/10/29}{Added macro}
 The |\nobreak| added to prevent null lines when |\\|
 ends an overfull line.  Change made 24 May 89 as suggested by
 Frank Mittelbach and Rainer Sch\"opf

Similarly in the final sources of latex2.09 (latex.tex)

% \nobreak added to \newline to prevent null lines when \newline
% ends an overfull line.  Change made 24 May 89 as suggested by
% Frank Mittelbach and Rainer Sch\"opf
%
\def\newline{\ifvmode \@nolnerr \else \unskip\nobreak\hfil
  \penalty -\@M\fi}

You can get this if the word ends with a hyphen:

\documentclass[]{article}
\textwidth=1mm
\begin{document}
margin-\\xyz
\end{document}