Mapa is not the only exception, actually! Words like clima, dogma, sistema, teorema all are masculine and end in -a. (plus some words that refer to people, like guardia or atleta, that can be masculine or feminine).
And there's also words like águila or hacha, which are feminine for all purposes except which definite article you use with them, because the two a's right next two each other in 'la águila' sound bad. Leading to constructions like "el águila calva" where the article is masculine and the adjective is feminine.
Ooh, good point. (I didn't see those words in Spanish class, pretty much ever.)
I also forgot "papa," which created a humorous situation in Miami:
When John Paul II went to Miami, an enterprising T-shirt salesperson made up shirts that said "I Saw The Pope" in English and Spanish.
Well, that's what they were
supposed to say. The Spanish ones used "la" instead of "el," which meant they said "I Saw The Potato."
