Undeletable mark--amazing properties

A mysterious, undeletable mark appeared in an Excel 2003 spreadsheet. It looks like a forward slash, but half-length and more sloped (45-deg.). It won't delete, but it copies to other cells and to an empty spreadsheet! (copy available). Writing in or pasting to the cell merely overlays the mark, and doesn't replace it. It does not show in the formula-data entry box (fx). Font color cannot be changed from black. It is left justified, independent of cell width, and won't format center-justified or right-justified. It remains after saving and reopening the spreadsheet. Any clues how to get rid of this?