To echo Malu: Americans who go to Mexico armed with an English-Spanish dictionary will sometimes root around in it when it's time to pay the bill at the restaurant and say to the waiter "Como mucho?" which is intended to translate as "How much?" It doesn't. The word "como" is the first person present tense of the verb "comer" which means "to eat." So instead of asking "How much?" the dumb American is saying "I eat a lot."
The waiter can either retranslate the bad Spanish and give the customer the bill for the meal, or be put in the position of either politely agreeing or disagreeing with the customer, which does not answer the question "Cuantos cuesta?" ("How much does it cost?" or "How much?") and can frustrate both parties.
And no, this didn't happen to me.
