Occurs on Russian Wikipedia, doesn't occur on English Wikipedia.
If the search query contains some word, which is by accident used as a namespace name, and a colon sign : anywhere after it, the search is performed in that namespace.
For instance,
Query: У холмов есть глаза: Начало (the Russian name of the graphic novel) (https://ru.wikipedia.org/w/index.php?title=Служебная:Поиск&profile=default&fulltext=Search&search=У+холмов+есть+глаза:+Начало&searchToken=7v64zbxyubf5yyw9dan118cma)
Expected result: Search is performed in the main namespace, with the possible outcome of the article about the novel being found.
Actual result: Search is performed in the "Участник" (User) namespace.
The obvious reason for this is that the word "у", which is a preposition in Russian, is an alias for the "Участник" namespace. Other examples of common words used as namespace aliases are "и" (and) and "к" (to). So, it's like if you searched "A Nightmare on Elm Street 2: Freddy's Revenge" on English Wikipedia, if "A" was a namespace alias.
The position of the namespace name in the query is irrelevant: it can be in the middle as well as in the beginning.
This bug can be quite painful, as it actually forbids entering several frequently used words as search items, without user remotely having a clue as to what is going on. It is also a huge headache when trying to find something with prefix: keyword, as a colon becomes present and the namespace names such as "Википедия" (Wikipedia) are frequently used in project talks (say, Википедия prefix:Шаблон: searches in "Википедия" namespace instead of "Шаблон" (Template)).
The same is true for the suggestions bar: if one types "У холмов есть глаза:", he gets pages in the "Участник" namespace.