Solution for "Illegal parameter number in definition of \test. You meant to type ## instead of #, right? when creating a latex table with double vertical line"

Illegal parameter number in definition of \test.

<to be read again> 
                   =
l.343 \end{tabular}
                   
You meant to type ## instead of #, right?
Or maybe a } was forgotten somewhere earlier, and things
are all screwed up? I'm going to assume that you meant ##.

Solution:

Use #### instead of #.

Problem:

Overleaf gives this error when creating a table with double vertical lines.

% THIS DID NOT WORK
\begin{table}[Summary Statistics of the analysis]
    \centering
    \begin{tabular}{|c||c|c|}
        \hline 
        1 & 2 & 3 \\ \hhline{|=#=|=|}
        4 & 5 & 6 \\ \hline
        7 & 8 & 9 \\ \hline
    \end{tabular}
\end{table}

This part is the cause of the error: 1 & 2 & 3 \\ \hhline{|=#=|=|}

Instead, we should do: 1 & 2 & 3 \\ \hhline{|=####=|=|}

% THIS WORKED!
\begin{table}[Summary Statistics of the analysis]
    \centering
    \begin{tabular}{|c||c|c|}
        \hline 
        1 & 2 & 3 \\ \hhline{|=####=|=|}
        4 & 5 & 6 \\ \hline
        7 & 8 & 9 \\ \hline
    \end{tabular}
\end{table}