Why does this happend when I render the bow in "firefly mode".

You need to uncheck "smooth polygons" for that figure. It's a known problem which affects some objects, not others. I haven't yet encountered it but it's reckoned to be a commonplace happening.

Coppula eam se non posit acceptera jocularum.

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ok thanks for your help. // Fredrik

In the render options switch off smooth polygons and only use it if you need to. Its the same with use displacement maps, if you arent using displacement then that option can be turned off too.

thank you xantor for your answer, yes I was needed to switch off smooth polygons in render options to get the bow to look as it should.