It occurs when two different versions of the film are fused together, usually in a Director's Cut.
In a bid to release a definitive English-coherent version with all scenes intact, this might mean inserting sequences only ever shot for the Italian market - hence the subtitles, since it is too late to dub over.
Many films were released with different edits between Europe and the USA (Dawn of the Dead, for example), with sex and extreme violence often missing from American versions but different scenes included to compensate.
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