it was coca cola mixed with coffee. sweetened both with high fructose corn syrup, and two artificial sweeteners, simultaneously. I still remember the aftertaste.. it's not something you forget.
I'll have like 1/4 glass of milk and add chocolate syrup then as I mix that I add seltzer and if stirring correctly it won't fizz up. I know people say adding the acidity of seltzer spoils the milk, but I've never noticed it when I make it.
Spoiling milk is all about the ratio of the fat. It's why cream is used in alcoholic drinks. The higher fat content of the cream means it can take more without spoiling. Try yours with skim milk vs. whole and taste the difference.