Korrd - 3:10 am on Apr 11, 2010 (gmt 0)
I had a site do that. It got a "-950" penalty in November and stayed penalized until Feb when it started yo-yoing in and out. Essentially it started getting released from penalty and taking about three days to bubble up through the SERPs, and then as soon as it got back to page one, smack, back to -950 for a few more days again. But eventually it stabilized and has been stable back at its pre penalty page one positions for the last month.
I don't know what the penalty was for, and I didn't intentionally do anything to get out. The site is for a real world business and is totally "white hat" so there was nothing obvious to fix. Changes were limited to regular content updates, which is normal activity for this site.
Personally I suspect sabotage although I wasn't able to find a smoking gun. But the timing coming right at the start of the busiest season and the specificity of the penalty to just two keywords sure pointed to it. The site was penalized only for searches containing the niche name or the name of the city in which the business is located.
The later is most telling as the name of the home city only appears a couple of times on the site. For that business in that city that's sufficient to rank and the name is not important anyway as the business supplies services at the client's location and the home city is a suburb of a much larger city. The larger city is mentioned prominently on the site but was not penalized for.
Plus there were suspects, starting with a SEO company that had cold called the owner wanting to sell them 500 backlinks and then followed up with a second call during the penalty. There were also a couple of real world competitors who would have had "motive and opportunity". One had been aggressively trying to poach the business's clients, the other trying to get the business to convert from independent status to becoming one of their franchises.
Both have been targeting to rank for "business niche + name of the home city". The poacher has a personal hatred of the business's owner and a spouse who is a SEO person (but not an expert). The franchiser quite obviously has some big time off page SEO behind their site to dominate the SERPs nationally the way they do.
The only other possibility I've been able to come up with but which I consider remote, is that the site had just lost a ten year old backlink due to the backlink site closing. This was the third ten year old backlink lost in the last two years. The other two, one site sold out to a competitor who deleted all the outgoing links and replaced them with links to their business, and the other was a hobbyist who threw up their hands "sick and tired of all the spam link requests" and removed all outgoing links from their site.
But in the end I still don't know what happened or how close it might be to recurring.