Exceptions to "no articles before professions" rule?

Answer 1

Add an article to a profession if it's qualified by an adjective:

(no article) Du bist Schüler. You are a pupil.

(article + adjective) Du bist ein guter Schüler. You are a good pupil.

Which brings us to the old Internet saw about President Kennedy at the Berlin Wall. He said, "Ich bin ein Berliner" which supposedly meant "I am a jelly doughnut (obviously, a particular kind of jelly doughnut named after Berlin). Yes, it might have been more grammatical had he said, "Ich bin Berliner." But judging from the crowd's reaction, his meaning was clear.

It might also have been taken as Ich bin e i n Berliner = I am one Berliner. But that's hardly necessary to consider.