In Russian language, letter 'Е/е' is often used instead of 'Ё/ё' (noted the two dots above the letter). Even professional sources, books and dictionaries may list, for example, "чёрная дыра" (black hole) as "черная дыра". The problem is, MediaWiki Search Engine treats these two letters as completely separate. It will return zero results when looking for "черный" if the article is called "чёрный" or contains the word.
Current workaround: article editors must always create articles with Е that redirects to Ё. But this workaround only solves the problem of looking for articles. Searching for text inside articles is still broken.