Blue diamond shape in a cell
 

By mistake I created a circular reference in a formula. When displaying the
spreadsheet on another computer a blue diamond shape appeared in one of the
cells affected by the circular reference. The cell is formatted to
Accounting with 2 decimal places and the GBP symbol. The diamaond shape
appears to the right of the £ with a space between them. I can find no
reference or help on such symbol/shape. I cannot recreate the shape on the
original machine. Can anyone explain what it is or represents? Thanks.