Where I work, a number of years ago a number of people swore they saw a ghost on the roof of the building. We actually installed a camera to try to catch video of it but we never saw anything out of the ordinary.
Years later, and after we started recording our cameras digitally, a user claimed we had captured a ghost on video. Upon review, it certainly appeared that way: a person suddenly appeared in the middle of a walkway, walked about 10 feet, then disappeared.
It turns out the cause had nothing to do with ghosts. In actuality, a bad Parallel SCSI connection was causing I-frame drops due to SCSI's tendency to throttle back the data rate when connections weren't perfect. It was originally U-160 (160Mb/s) but would throttle back to 80Mb/s or even 40Mb/s. At the time, we were recording that server at ~70Mb/s.