this one does not compile:

\documentclass{article}
\begin{document}
\begin{tabular}{ll}
a & b \\
[c] & d \\
\end{tabular}
\end{document}

if required, one can specify the vertical gap between lines in a table using
\\[len]

LaTeX removed the newline and whitespace to interpret the table as having a \\[c] entry, and there exists no length "c".

So the solution is to write the initial square bracket into curly braces. This way it is not interpreted as the beginning of the optional argument of the newline command.

\documentclass{article}
\begin{document}
\begin{tabular}{ll}
a & b \\
{[}c] & d \\
\end{tabular}
\end{document}

Thanks to Werner Grundlingh for showing me the problem source.