(Or a fix for this.)
I've found it can sometime mess with emotes (using underscore or star), and also (more annoyingly) when you double-click a word to highlight it and apply bold/italics it always highlights a space next to it, and the current system will break if there is a space as part of it.
Well, double click highlights the word and the space after it. Hitting Bold or Italics in the formatting bar just adds in *'s or _'s around whatever you selected. I'm not sure that this is something riot can fix, or if that's just how double-click-to-highlight works.
On the topic of smilies, you can use the escape character, \, to make them. Just use \ before every special character, so:
\*\_\*
Will always turn into
*_*
