The first time I travelled to Shanghai, I was walking down the street, simply wandering around. I walked into a small clothing shop to browse around and came across a great shirt that I immediately loved. I went over to the shop owner and began my first conversation in Chinese with a real local.
I said: "wo yao mai na ge chenshan" (I want to buy that shirt). The problem? I said 卖 (mài, to sell) not 买 (mǎi, to buy). So here I am, a foreigner in this tiny clothing shop on a hutong in Shanghai, getting in an argument with this shopkeeper telling him "I WANT TO SELL YOUR SHIRT" and he's saying "NO! YOU CAN'T SELL MY SHIRT".
Finally, about 10 minutes of argument later, a kind Chinese lady (who seemingly had more experience dealing with terrible accents than the shopkeeper did) came over and informed me of the confusion I was causing with my wrong tone.
I profusely apologized to the shopkeeper and was incredibly embarrassed. It was a great first "local" experience and gave me the incentive to always try to improve my language, regardless of the embarrassment.