One of the lexical similarities between reconstructed Proto-Indo-European and Proto-Uralic is in the interrogative and relative pronouns. For the former, in PIE there's a family of interrogatives beginning *kw-, e.g. Latin quis 'who?', quo 'whither?', while on the Uralic side there is e.g. Finnish ku-ka 'who?'. For the relative, there's PIE *yo-, giving e.g. Greek hos '(he) who', while Uralic shows e.g. Finnish jo-ka 'who'.
Proponents of the Indo-Uralic hypothesis see these as cognates; opponents see them as borrowings from IE into Uralic. It seems bizarre that such items could be borrowed, but stranger things have happened... Or have they? Are there any attested cases of a language borrowing an interrogative or relative pronoun?