The expanding use of key-val syntax in class and package options
means that packages can break due to unknown values. 
I just tumble over this example in a real document:

\documentclass[index=totoc]{scrbook}
\usepackage{acro}
\begin{document}
\end{document}


!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! LaTeX error: "kernel/boolean-values-only"
! 
! Key 'index' accepts boolean values only.


It is easy to provoke a similar error with e.g. siunitx:

\documentclass[mode=special]{scrbook}
\usepackage{siunitx}
\begin{document}
 blub
\end{document}


I'm wondering what could/should be done to avoid such situations.
Should package authors avoid package options altogether and use
setup commands? Or can they (how?) parse the keys in the option list
in a way which doesn't gives errors but imply ignores unknown
values? Should the documentclass filter the options? How? 



-- 
Ulrike Fischer 
http://www.troubleshooting-tex.de/