Oh boy... that's pretty scary.
To deliberately introduce security holes, sometimes minor changes are enough. For example replace "==" (comparison sign) by "=" (assignment). These "attacks" are visible to a trained eye.
But what happens if the eye can't see anymore? With Unicode, it is possible to use characters that look like our Latin alphabet, but are not, or worse change the writing order (left-right) so that the text is displayed one way in the text editor, while the compiler will interpret it differently. This opens up the possibility of inserting security holes that are almost impossible to see, even if you have the source code in front of you in your text editor.
(For an example of left-right inversion, go to this page: https://sebsauvage.net/wiki/ and look for my email address in the page: It shows up normally, but if you look at the html source, it shows up as a different text).
I think it would be interesting if text editors had an option to display in a particular color everything that is not purely "Latin text" (0000-024F), as well as Unicode characters that cause changes (backspace, change of direction).
Proof-of-concept of this attack in different languages can be seen here: https://github.com/nickboucher/trojan-source
We can develop an extension for each editor that highlights these characters easily !