Could of, would of, should of, might of.
As seen here.
HAVE, people. Could have. Could've. Not of! Few things make me think an author is an idiot more than seeing that particular error. Granted, in the above example the offensive section heading might have (HAVE!) been entered by an error or layout person, and may not actually be the fault of the author. But still. Bleagh.
Update: The above example was corrected as I was typing this post--possibly a flood of email from fellow grammar grinches brought it to the attention of editors. So, good for MSNBC, I guess--they didn't catch it before it was published, but did fix it pretty quickly.