Counsellor

Technically, you're right -- grammatically it should be "ein Mass".

However, in Bavaria (or maybe just in Munich), when filled with beer, the "Mass" ("measure") becomes weiblich (feminine) and thus "eine Mass, bitte" is what one orders.

I was told that this error in gender (as well as people eating the Weißwurst the wrong way) are ways true Münchners tell the tourists apart from the locals, even if you are wearing Trachten (Lederhosen and Dirndls).

Maybe a true Münchner or Münchnerin can tell us whether I've been hoaxed.