Spellcheck has changed the way we all proofread our writing. While even editors love spellcheck (whether they admit it or not), it has made all of us a bit less diligent as we rely on those squiggly red underlines to let us know when we've transposed letters or made use of "creative" spelling.
Below are some of the most common errors I see that spellcheck won't catch. While some require no explanation, I've offered some hints for understanding the trickier of them:
affect/effect - These two words are frequently confused. Typically, affect is a verb (to influence something) and effect is a noun (a result or consequence): The weather affected our plans. The medication has few side effects. However, English being the malleable beast it is, both can function as either nouns or verbs. Affect (n.) is used to describe a mental or emotional state (She had a pleasant affect). Effect (v.) is to cause something to happen (The legislation did little to effect change). Affect (n.) is rarely used in legal writing, so you can generally test for affect/effect by replacing the word with another verb to see whether the sentence still makes sense. If so, use affect.
born/borne/bourn - Most commonly, contract drafters use born when they mean borne. Only use born when discussing an actual birth: Babies are born; costs are borne. A bourn is a small stream, and it is unlikely you'll spend much time discussing small streams in your legal writing.
choose/chose - Choose is present tense; chose is past tense.
its/it's - It's is a contraction of it is. Its is a possessive pronoun. Contractions are too informal for contracts and other legal writing, so it's should be avoided. Just be careful you aren't shortening it is to its.
than/then - If you have a hard time differentiating between these two (as many second-language learners do), try remembering that then tells you when something happens.