No method has a nonzero error rate; countermeasures and bad luck produce false positives and false negatives::
We match on keywords: they misspell (V1AGR4, pr0n)
We use word frequency, other stats: they include random text
We scan text, they put their text in pictures
We use OCR on their pictures, they muck up backgrounds
(actually FuzzyOcr helps) - In the next round, who knows?
The latest: ASCII art plus ramdom text.