One of the koi in our pond had been acting strangely recently, swimming around on its side.  It was one of the larger ones, with orange and white markings.  Well, Krista found it laying on the bottom of the pond.  We used the net to get it out of the pond, and it was dead.  We believe it may have received a small electrical shock (possibly from the pond lighting), which eventually caused its demise.  All of the other fish are fine, although we're watching them closely to see if any of them experience the same problem as that would be a cause for concern and could indicate some sort of fish disease.  Hopefully that won't be the case though, and the rest of the fish will be fine.