I'm generally forgiving of homophone misspellings -- in fact, one of my pet peeves is the way language scolds like to label such misspellings "confusions," suggesting that people don't know the difference between then and than or loose and lose when the misstep is plainly (in context) a matter of spelling. The right word is intended -- nobody confuses the meanings of then and than -- but the letters are wrong.
The tougher cases come with words whose meanings can overlap significantly. Disperse for disburse is fairly common, and often can't be ruled absolutely wrong; party hearty and party hardy are both plausible; even reign in for rein in often makes a sort of sense. (As in the campaign literature of a just-defeated candidate for Mass. State Auditor who promised to "reign in wasteful political spending.")
And here's one from today's Wall Street Journal article on new FDA-approved technologies to zap body fat. Mitchell Levinson, chief scientific officer of Zeltiq (which freezes away fat cells), is quoted as saying "This is for patients who have a discreet bulge they want to get rid of." But wait: If the bulge is a "discreet" one, why pay thousands to reduce it?
I think Levinson, man of science, said discrete. But in stories about liposuction, plastic surgery, and the like, the word discreet is much more common -- roughly twice as likely in raw ghits. And it was that context, I suspect, that made discreet look OK to the writer or editor or both. (That's my theory, at least. Now I'll go ask Melinda Beck, author of the otherwise excellent article, whether I've guessed right.)