I was an undergrad that did research over the summer at the MIT campus for LIGO, which is a 40 meter test beam. Sam Waldmann (now at SpaceX) and Rai Weiss were my two main advisors at the time. Part of my job there was to gain a lot of new experience with electronics, circuit design, and hardware development before spending my senior year doing a thesis on LIGO. The thesis focused on studying and installing the quadruple pendulum system ("quads") that helps passively isolate random noise in the mirrors themselves. However, during my summer, I worked on the Internal seismic isolation platforms ("ISIs") which are actively dampening seismic noise from the environment. As you might guess, these included an array of sensors and seismometers measuring the input and applying a transfer function to determine a correction given the known characteristics of the system. After a month working hard on putting together test boards that could be hooked up to the seismometers on the ISIs, it was finally time to test the seismometers when they came in. So I went into the clean room, along with my graduate student, and hooked up cables from the sensors to the computer -- through a makeshift hole in the wall separating the two rooms. Graduate student left for lunch and I decided to quickly turn the computer on, turn on the hardware, and see if I could get a signal. I saw nothing initially and thought I screwed things up, so I panicked, unplugged, and replugged. All of a sudden, I saw a big spike on the screen across all sensors and thought I broke something -- and then the room started shaking.
I inadvertently recorded the 2011 Virginia earthquake from Cambridge using seismometers installed on one of the ISIs for LIGO... whoops!