The Al Chet that we say four times on Yom Kippur - the long list of sins that we list and ask to be forgiven for - is a double-acrostic (in nusach Ashkenaz). Each letter is represented twice in a row (aleph, aleph, bet, bet, etc.) except for samech (ס), which is replaced by sin (שׂ). I would say it's a cute pun in English (sin/sin), but there must be a more believable reason for this.
Why do we replace samech with sin in the Al Chet?