Some months/years ago, TIm posted some useful tips for testing Moodle.
I think these should be directly added on the QA testing documentation page, to encourage most people to take care of that when testing Moodle.
Certainly other tips could be added, like trying to use (very) long strings, with simple and double quotes, ponctuation characters... or try using several languages.
Or these different aspects are already covered by automated tests?
Séverin