When I was growing up I always thought that the term black market refered to an area in some dark, secluded and sinister place where all the buildings were painted black and where people of a darker shade did their shopping (I grew up at a time where Zimbabwe had only just come out of a system that polarised on the basis of colour). Along with other erroneous childhood beliefs it went out the window when someone explained the truth to me. The real problem lay in the words. Then there are words that can be easily confused with others e.g. mastication-the act of chewing and ...well you get the idea right?
If you want to expand your vocab take a look at Dictionary.com where you can sigh up for a word a day email to help avoid all those embarassing situations.