Google yanked a site of mine from their index once. I played what they thought was a "dirty trick" to get a better ranking, but I resolved the issue and am now listed again. I employed the age old method of using white text on a white background to repeat keywords over and over again. A bot or crawler of theirs discovered this, and they emailed me and told me I'd been removed.

I appreciate the way that Google tries to play fair, but I wish they'd make what is off-limits more widely known. My trick was bush-league and easily traced. What else to they have bots looking for? What else, that may not be immediately obvious, will they pull you out of the index for?