"A shit" instead of just "shit". Is this common usage?

I think the awkwardness of the phrase is supposed to add to the humour, like Liz Lemon saying “I want to go to there”.

Well, that may be a very good point. Was the awkwardness of the phrase the intent of the writer/director???

I saw Ted in the theater and the moment I heard the line I thought it was strange. When Mark Wahlberg came in and repeated “is that a shit” I heard someone behind me say “a shit?”, confirming that I wasn’t the only person to find the line odd.

Is it possible that the wordage of the line was intentionally unusual to focus humor of the situation and/or to engage the audiences attention?

But you certainly can ask for “a water”, as well as of course “a beer”, and even “a ketchup”. If it’s a standard and familiar serving or container-full, then we eventually skip the “container of…” and refer to the serving as a singular noun.

Personally, I don’t give a shit.

“A shit” instead of just “shit”. Is this common usage?

This question has profound implications.