This question already has an answer here:
Similar to this, but containing more bugginess.
Take a look at this:
The post was grayed out because it received (presumably) 3 offensive flags. The answer was simply a random string of profanities.
Update: In some situations, it can even appear below other answers making selecting a flag option impossible. The only workaround is to drag the box -- using the top right corner -- to whitespace on the right side. Then the post may be flagged.