discreet: prudent, especially capable of preserving prudent silence
discrete: constituting a separate entity, individually distinct
Learn the difference, people. Yes, I'm talking about you, Scott Adams.
While I'm at it:
principal: most important, consequential, or influential
principle: a comprehensive and fundamental law, doctrine, or assumption
I'm sure there are other frequently misused homophones that annoy me, but those two are the worst.