Seth Finkelstein has made a startling discovery, backed with lots of evidence: when a spam result comes up in a search, Google not only blocks the spam, but every result after it. This means that for searches where spam results manage to rise to the top, very few -- sometimes zero -- results will be returned.
Examples: motorcycle candle, keyboard bracelet. These are common words for which you'd expect to see results (in fact, the latter even says there are thousands of results) but the bug prevents them from being seen. Eliminating the spam pages (by blocking a word, antenna, that appears on them) allows full results to be displayed.
It's pretty amazing that such a serious bug made it past Google's tests. It will be interesting to see how quickly it's fixed.