in LaTeX code @ should be treated as a letter, any number of examples on tex.stackexchange but
Undefined control sequence on \beamer@leftmargin indentation
\begin{frame}[fragile]{E}
\makeatletter
\hskip-\beamer@leftmargin
\makeatother
\lipsum[2]
\end{frame}
\beamer@leftmargin is a single token but beamer is coloured and @leftmargin is left as unstyled text which makes the code very hard to read.
Technically @ is not always a letter but it is almost always a letter when appearing in code sections and is a far better default in a syntax highlighter.