Yeah I was using Flickr, just browsing photos at no particularly fast rate (and I have no spyware issues or anything), and got the 999 error as well. This is pretty ridiculous and is the kind of thing that can be almost suicide to a website--think about it, if people get pissed off from not being able to access a site, they'll just take their business somewhere else.
Something is wrong with this--flickr/yahoo needs to either remove this or adjust it or something, because it is really annoying. Keep in mind, that if just one person causes a 999 error to get issued, then EVERYONE who is on that IP will also be blocked for some period of time. If your ISP uses a NAT then it could affect a lot of people who are going out on that ISP NAT address, if that is how flickr/yahoo is determining which IP to block.