I went to Mexico and told a lot of people that they don't speak Spanish. When someone said something that I couldn't hear well, I'd compliment their digestion.
Right phrase: No hablo Español (I don't speak Spanish) What I said: No hablas Español (you don't speak Spanish)
Right phrase: Que dices (What you say) What I said Que diges (what digestion!)
That's a really common one here in San Antonio where people speak conversational Spanish but don't know more formal words like that. One of my friends tripped and fell in front of her family visit from Mexico. It was at our graduation and she stepped on her gown. She said "estoy embarazada" and everyone freaked out, running to her aid. Her parents were really confused how their lesbian daughter got pregnant.
Because people are super accommodating in non-WASPy countries. I'd try to speak Spanish and if they saw I was struggling too much, they'd say "let's speak English."