Confusing word pairs

How many confusing word pair can you name? Let's see, there are qué and cuál ("which" or "what"), por and para (often meaning "for"), de and desde (usually meaning "from"), and saber and conocer (both translated as "to know"). And there are others among the lessons on this site as well. If you know of any confusing word pairs that we should have a lesson for but don't, please write, but use the search boxes first to make sure they haven't been covered already.