Ennui (I thought it was pronounced en-you-eye, I thought on-wee was a french name spelled like Henri. I was making this mistake up until a year ago actually)
**Hyperbole **(10th grade English sorted me out there, though)
**Milquetoast **(seriously why is this not spelled milktoast?)
Also, on a similar note, for years I thought that the differently-pronounced versions of the word “separate” were spelled differently [verb (SEP-uh-rate) spelled “separate” and adjective (sep-uh-RIT) spelled “seperate”]. All because of a typo in a Babysitters Club book I read when I was 8.