I also have a weird speech impediment that causes me to replace s's (and some c's) with f's and v's, depending on whether it's a voiceless alveolar fricative or a voiced alveolar fricative, respectively. This is usually when the sound is at the end of a word, most often words like yes (yef) and is (iv). I was not born with it, and I do not do it on purpose.
....probably should mention that I'm a student of languages and sound...
And I know my handwriting is crap, so this is a trailer for the movie I watch whenever I'm sad. [link]
Watch it, love it. Yoo Ah In is adorable, and so is Choi Ji Ho. i like my new style so much better than the old one.