I had some problem with the Oplev Servo today. I was working at the mode matching fine tuning and I left the Oplev servo enabled while aligning.
When I opened the Yend table lids, the Oplev beam started moving on the QPD and the Oplev servo didn't help in stopping the mirror movement, but it increased it.
So, the mirror was oscillating at a frequency of a few Hz
Koji suggested that maybe the shaking is due to the air conditioning moving the beam, so the servo tries to feed back the signal to the mirror, even if the mirror doesn't actually move.
I also measured the transfer function for the Oplev, but it didn't show any strange behavior.