Eureka!

The problem is that Custom data validation was turned on (with no formula
entered). Turn off data validation and the problem goes away. That leaves
me with another puzzle. Using the menu commands, you cant set Custom
validation without entering a formula, and I certainly didnt do it in a
macro. Since my application was built from an inherited workbook, anything
is possible. The smart tag flyover message on these cells that says, €œThis
cell contains a formula and is not locked to protect it from being changed
inadvertently€� is a wonderful piece of half-truth that helps obscure the
issue. Apparently the smart tags never heard of data validation.

--
Al C