When I worked at Mozilla, on MDN, we had this mysterious bug:

https://bugzilla.mozilla.org/show_bug.cgi?id=776048

The tl;dr is that MDN is a multilingual site, and supports categorization in a few ways, including by tagging. An example of the bug: the English MDN's CSS reference tagged articles with "CSS Reference". The French MDN's CSS reference tagged them "CSS Référence". Sometimes, the French tag appeared on English articles.

The source of the issue was MySQL's utf8_general_ci collation, which did not see "e" and "é" as distinct, so it was a toss-up as to which tag you'd get back from the database. The solution was to build and install a custom collation to teach MySQL to see accented and unaccented characters as distinct.