Not precisely a homonym, but my current least favorite in that vein - due to encountering it so often, in both directions - is "affect" vs. "effect".
The trouble is that both of them are valid as both verbs and nouns, in the same grammatical context, but they have such different meanings...
The trouble is that both of them are valid as both verbs and nouns, in the same grammatical context, but they have such different meanings...