I have always been super careful with using any slang, unless I completely have a grasp of its proper use in both cultural and linguistic context. better safe than sorry, as a lot of my slang in spanish was from mexico and central America before I came to Chile. some of the more colorful slang from mexico, Chileans were really shocked at, as it has much stronger cultural connotations in Chile.
Then I finally started driving in Chile, and the first time some weon cut me off in traffic, it just naturally came out. I turned to my wife and said, "I finally figured out how to use it correctly".
One of best moments of "misused slang" was riding from Guatemala across Mexico back to the United States with a friend from Canada that had lived in Guatemala for many years.
We got pulled over by a local cop in southern mexico. If you have spent any serious time in the rural parts of Mexico, that has the potential to be big trouble if not dealt with the right way. My friend had this really low class teenage girlfriend / wife from Guatemala city that had taught him Spanish.
well, the first thing my friend did, in an attempt to be friendly with the cop was to essentially say in Guatemalan slang, that translated to Mexican slang, 'would you like a BJ?
The cop cracked up laughing, and my friend's sister with a much better grasp of mexican spanish translated what he had just said for him. His face went white, and he started apologizing to the cop as fast as he could. The cop could not stop laughing, and when he had caught his breath, told us to have a good day. We obviously were not some criminal masterminds. In the rearview mirror I could see that cop still laughing as he walked back to his car.