Another entry in an irregular series.
past and passed. In the context in which they're often confused, past means "beyond" ("He looked past her"); passed is the past tense of the verb pass. "He looked passed her" is incorrect. (I sympathize with people who get this wrong; in recent years I've found myself accidentally typing homonyms (and not noticing I've done so) more and more often. I never used to do that. My brain is making cross-connections where it shouldn't. I know the difference between their, they're, and there, but my fingers sometimes type the wrong word. Embarrassing, especially when it happens while I'm typing a query or suggestion as part of editing a story.)
taut and taught. In several stories recently, I've seen phrases like "She held the rope taught." Taut means "stretched tight"; taught is always a verb, the past tense of teach.
pored and poured. To pore over a book is to read it carefully. Whenever I read "She poured over the book" I think of maple syrup being poured over pancakes.
nock and knock. To nock an arrow is to fit it to a bowstring. (Derives from the same root as notch.) You don't knock an arrow under normal circumstances.