Internet Explorer is throwing up an error that the file you have just rendered can't be downloaded.
IE states file cannot be downloaded
Explanation about a random error message occasionally provided by Internet Explorer when attempting to download a render
This happens apparently randomly when you are saving renders with snapshots. It is a timing issue between IE and the file system when you save the same render into the same place repeatedly.
If you wait slightly longer before you save an identical file over itself you will find you are not troubled by this error.