I know that at times I can be a very petty person. I have always been mildly annoyed by the phrase, "a coffee" as in , "Let's go get a coffee." I have always alternated between, "Let's go get some coffee", or "Let's go get a cup of coffee." If I'm going to a cafe or pancake house, I'm expecting a bottomless cup so there will be much coffee drunk. Since they keep coming by at various phases of the coffee consumption, it is impossible to determine how many cups are consumed. We're just drinking coffee until we place our hands over the top of the cup to indicate that we are now finished with our conversation, hence the coffee drinking will end. That is the time to use the phrase, "some" coffee.
If we are going to a coffee shop or a Starbucks, the expectation is that the drink you order is the drink you get. No refills, no second order. You never hear anyone say, "I'll have another Grande Americano with an extra shot and room." It's considered bad form. Hence, the proper phrase for that event is going for a "cup of" coffee. One cup, and when it's gone, the experience is over.
When someone says, "I went across the street for a coffee", I immediately tag them as a coffee lightweight. Or they may be trying to sound European. Either way, I will not consider them true cafeteros. Maybe I'm being petty and judgemental. Maybe I'm just being regional and the offending phrase is perfectly acceptable on the east coast. Or maybe I'm just sick of having my agenda altered by snow and have grown grumpy.