another interesting twist when using breqn package. Now this fails $y=x y'+y'^{3}$ with ! Double superscript error. But without breqn, no problem at all., This must be known thing. But easy to fix. I changed it to $y=x y'+(y')^{3}$. MWE
\documentclass[12pt]{book}
\usepackage{amsmath}
\usepackage{breqn}
\begin{document}
$y=x y'+y'^{3}$ %gives error with breqn only
\end{document}