Hi,
Maybe a bad connection or time-out on your connection. If the mail is not retrieved, it will stay at your ISP for you to pick up at a later time.
In short, let fetchmail try it at it's regular interval. If the message is comming up too many times then it's time to check your connections, DNS or have a serious talk with your ISP.
In short, it doesn't husrt once in a while, but keep an eye on it

HFW