I've been mixing grain and grapa.
There is not enough information about this idiom on the internet. The idiom means that one is mixing (drinking) different kinds of alcohol which should lead to a hangover. I've seen also these wordings: "a grain and a grapa," "the grain and the grapa," "grain and grappa." And I have not find a single usage on Ngram. (Interestingly the word grappa is becoming more and more popular since 1920).
What is the standard usage and would people understand it today? (Can you give an account so when someone googles it they find this page.)