#5 \marginpar does not work: You can't use `\end' in internal vertical mode.

[Martin Schröder]

\marginpar does not work:

\documentclass[crop=false]{standalone}

\begin{document}
Lorem ipsum
\marginpar{Testing}
\end{document}

This leads to the following error:

You can't use `\end' in internal vertical mode.

This also creates an incompatibility with todonotes, see here.

[Martin Scharrer]

Yes, marginpar and similar things are not supported.
standalone is not intendend for normal running text, but for diagrams, single paragraphs etc.
It does not provide a normal page setup with the text box and margins, but captures the content in a box. The options 'crop', 'preview' or 'varwidth' basically just add a paragraph box around the content before boxsing it to allow for vertical content.

If you want to have running text with marginpars then you would need a different class or package. Maybe 'subfiles' can help you.