Well - I've at least learned that diacetyl exists! After nearly 40 years imbibing, reading about and brewing beer, I can say with some certainty I'd never come across it before. Is it a new term for an old problem? As for taking beer back - a judgement call for me, in the same way as a complaint about food orany other aspect of an evening out would be. Risk hassle or just leave? Mostly when a pint's gone back it's been replaced without a problem though. But one time at a pub in Cambridge where the landlord was acclaimed for his range of real ales, he practically had me tarred and feathered at the bar for suggesting anything could be wrong with one of them. And right now I suspect you're thinking - perhaps ericonabike's taste buds aren't all they should be - whcih is why it's a judgement call...