Here's a really weird one. The following math notation looks wrong (mind the order of the brackets), but compiles into LaTeX

\documentclass{article}

\begin{document}
\[
\Big{(a + b \Big)}^2
\]
\end{document}

It fails in KaTeX, and really I can't blame KaTeX.

What's going on in LaTeX? \Big is a command that accepts one argument. As usual, if no brackets are here, it's just the first thing it sees, it many cases a bracket like \Big(. In the "typo" above, however, it takes the whole (a + b \Big) as argument, but it really only acts on the first bracket in there.

If you want to improve LaTeX compatibility, this could be fixed.