What foods and drinks should not be mixed? (Possibly NSFW)

Orange juice and milk.
Any beer and Coors.
Any alcohols.
Be wary of gin. If in doubt, use vodka instead.

Kim chee and absinthe.

Unless you’re already nude and surrounded by strangers – Then it’s perfectly okay.

I’ll just say it’s not pretty if you vomit a mixture Cheetos and Pepto Bismol.

^^ The thought…EWWWWWWWW!!!!!!!

Beer and any liquor
Mixing different liquors
OJ and milk (especially on an empty stomach)
Watermelon and milk (according to my parents)

Did you mean literally mixed in the same glass or bowl (like, say, tea and milk)? or mixed as in having them together at one sitting (like, say, tea and toast)?

@Jeruba Together in one sitting.

Well, I don’t know about “should,” but I wouldn’t care to consume or serve sweets with wine or beer. A birthday cake, for instance: no champagne for me. I’ll take a glass of milk, and save the champagne for later.

A company I used to work for had an afternoon event once a month during nice weather, called (and featuring) beer and cookies. That sounded seriously unappetizing to me.
 

Waiting—naively, perhaps—to see how this Q attracts NSFW answers.

^^ He marks everything as NSFW just to cover his butt…At least this time, he added the word “possibly” to the beginning of the NSFW!!! ;}

Giggity.

Beer and fruit. Especially raisins. Especially for breakfast.

Just thought of another. Never give a drunk coffee with cream. It curdles in their stomach and comes back faster than it goes down. Black coffee is fine