sidenotes package : error in figure alignment

You are a victim of end of line space. Put a % after [htbp]

\begin{figure*}[htbp]% %\centering \includegraphics[height=180pt,width=350pt]{example-image-c}% \caption{An even larger rectangle. This is the widest figure option. Both, the text as well as the margin width are used for the diagram.} \label{rectangle31} \end{figure*}% 

Full code:

\documentclass[12pt]{scrbook} \usepackage{blindtext} \usepackage{sidenotes} \usepackage{mwe} \usepackage[showframe]{geometry} \geometry{paperwidth=170mm, paperheight=240mm, left=42pt, top=40pt, textwidth=280pt, marginparsep=20pt, marginparwidth=100pt, textheight=560pt, footskip=40pt} % \begin{document} % \begin{marginfigure}% \includegraphics[width=\marginparwidth]{example-image-a}% \caption{A small rectangle put in the margin.\label{rectangle}}% \end{marginfigure}% % \blindtext % \begin{figure*}[htbp] %\centering \includegraphics[height=180pt,width=350pt]{example-image-c}% \caption{An even larger rectangle. This is the widest figure option. Both, the text as well as the margin width are used for the diagram.} \label{rectangle3} \end{figure*} % \clearpage % \begin{marginfigure}% \includegraphics[width=\marginparwidth]{example-image-a}% \caption{A small rectangle put in the margin.\label{rectangle2}}% \end{marginfigure}% % \blindtext % \begin{figure*}[htbp]% %\centering \includegraphics[height=180pt,width=350pt]{example-image-c}% \caption{An even larger rectangle. This is the widest figure option. Both, the text as well as the margin width are used for the diagram.} \label{rectangle31} \end{figure*}% % \end{document}