One thing that tripped me up with my PDP-11/23 was the aux switch on the front panel. I assumed it simply controlled the auxillary AC power plug on the back, which is what some of the documentation says. You might use it to power an RL02 drive, for example.
I didn’t realize that it can optionally control the line clock interrupt. My system suddenly started acting very strange, and was no longer able to boot XXDP via the TU58. It turned out I had flipped the aux switch, which was setup to control the line clock interrupt. After some help from the cctalk mailing list and the documentation, I found that the BVD11 can control the line clock interrupt via software if you set a certain jumper. Now I just leave the aux switch alone!