RubyMine comes with handful of mnemonics which are supposed to boost your productivity. However ones that are combined with alt keys can sometimes cause unexpected behaviours since some of them are bound to special characters (e.g. ™, Ó, µ …).

You can disable special characters by choosing “Unicode Hex Input” as an input source (System Preferences > Keyboard > Input Sources > Others).

Now typing Alt + key won’t interfere with any keyboard shortcuts. Happy RubyMining!