Slightly odd one - but my brother had his 123d in for its NCT a fortnight ago, and it was failed on having tinted windows. Specifically, the % light that they allow pass through. Its the third NCT the car has been through here, and first time its ever come up.
They're not particularly dark - and having just checked the spec sheet, it looks like they're actually factory fitted. Anyone (a) know what the standard tint % used by BMW is? and (b) anyone ever had this happen to them?
I'm guessing that BMW tint the glass, and don't use film - so its probably not the most straightforward thing to resolve if the stand firm on it.