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}