In Russian Wikipedia, there are two standard forms of User: namespace: Участник: for male users and Участница: for females. If you'll pass a pagename with wrong gender to MediaWiki API, it will be normalized into the correct form. But if you'll send a request that should return some pagenames, it is not guaranteed that those pagenames will be returned in correct (standard) form. So, if you have a bot that gets the list of user pagenames from API and then, for instance, asks API to return the list of categories of those pages, it will probably fail while parsing the result, because you probably will not parse normalized section assuming that pagenames from API are always normalized. My bot that sorts unreviewed files in Russian Wikipedia (https://github.com/Facenapalm/NapalmBot/blob/master/scripts/sort_unrev_files.py) has met that problem.
Unfortunately, I can't illustrate it with simple example because the problem is a bit random and I can't catch it with small requests. I've met this problem with complicated request with generator, two continues and more than 90000 results. Here it is:
In my case, one of 90000 returned pages was Участник:Miss Amber. Then I asked for its categories with request like that and failed to find this page in answer:
The problem appears between 29th of June and 6th of July. My bot runs this script every week at fridays; it runs successfully at 29th of June and fails every week after that.
I've already updated my bot so it now parses normalized section to understand corrected pagenames. But the expected behaviour is "API should always return results in normalized form", and it is broken anyway.