Denialism blog has the details, so I'll just outline what happened with links.
First extremists flooded a researcher's house doing around 20,000 dollars of damage. They had stated that they had been trying to decide whether to flood or fire the house. Well, they came back and set her home on fire after all.
In another story, animal rights extremists attempted to break into the home of a researcher while she was throwing a birthday party for her daughter.
As Mark Hoofnagle put it
The fundamental problem isn't the absence of appropriate safeguards and regulation of science, we have plenty. The problem is an ideological movement that lies about how we do research, and the scientific basis for using animals in research. It is classic denialism.The worst part about these extremists is that their actions actually hurt the cause of animal rights. There is room for discussion within our society about what level of rights we are willing to extend to animals and what not. But when animal rights extremists resort to these kind of terroristic tactics (activist commenters at Denialism said Hoofnagle deserves to die) they have said that there is no such space for debate. And shutting down that discourse does more harm to animals than anything else.*
*Edit - "The worst part" is a figure of speech that I typed out without actually thinking much about. The worst part is obviously the effort to threaten and intimidate researchers.