I can't tell you how many times I come into work and my computer is rendering something. For example, we had Thanksgiving day and the day after off, so the schedules for rendering on all computers were changed to allow rendering during work hours on those days...but whoever changed them did not change them back. So, this Thursday, I came in to work and my computer was operating very slow. Turns out, I had an extra 3dsmax.exe running in the background rendering a scene in the queue.
So, if you're hooked up to a render farm, check your schedule! Also remember that even if your computer shows 'unavailable', it might still be working on the last frame submitted to it before going off the farm:)