Hello,
little video : transparency bug - YouTube
The curtains have the double sided option checked (does the same with or without). It also happens in another area (with the same objects).
There’s a gap between the objects :
Any idea? Thanks !
It’s happening because glass material (window behind the curtain) also have some transparency. Order of transparent objects is rendered differently due a position of the camera. That’s why objects are overlapping when you are moving. the only way to avoid it is to make one of the materials not transparent - glass or curtains.
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Hi Magda, thank you for the precise answer ! Good to know for the future! 
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