It's pretty rare, I see it pop up once a week or less.

A handful of sites are blocked, after a while of being available iphonedevsdk.com was blocked, for instance. That makes perfect sense given they were the vector for lots of high profile attacks.

Noisebridge.net was blocked, but while organizing an event I requested it be made available and it has been since.

I suspect there is a lot of overlap with Google's unsafe sites list, but this was listed as "Education;Spam/Illegal".