This has been happening in some larger (5MB+) files recently:

When I refresh a pivot table on one sheet, a chunk of cells in my source
data table on a separate sheet is erased (as if I used "Edit- Clear- All"
on them). The dimensions of the range that is deletedi n the source data, is
identical to the dimensions of the pivot table (e.g. if my pivot table sat in
range A3:F100 on "Sheet1", my source data on "Sheet2" would suddenly have
cells A3:F100 cleared when I refreshed the table).

Any ideas?