Even worse, we actually had a regression at work caused by a zero width space sneaking into some code. Most editors won't even display anything for that character, even the code review tool we used didn't!

In the end I configured Emacs to highlight all unicode characters, trailing whitespace and tabs bright red. I also had it render zero width characters as normal spaces, otherwise they were still invisible. https://github.com/kzar/emacs.d/blob/master/init.el#L181-L19...