No, they aren't.

To answer the other respondent's questions, there are no macros in this
worksheet and setting the macro security level to high and to medium did
not solve the problem.

Even more interestingly, I then did a Ctrl-A Ctrl-C, got the usual warning
from Excel that the picture was too large and would be truncated. I pasted
this selection into a blank spreadsheet, did some formatting of column
widths and saved the sheet. The problem does not appear in this new sheet,
even after closing Excel and opening the worksheet several times.

The Excel sheet is a simple no-frills spreadsheet of receivables for my
little home business. All it contains are invoice numbers, payees, date
invoiced, amount, date paid and amount, with some conditional formatting
which shows the outstanding unpaid invoices in bold red. At the top far
right is a cell which keeps a running total of all unpaid invoices with a
window split to keep the first row on the screen while the rest of the
sheet scrolls.

Color me confused,

Tom