Formula copy error, when not copying formula

TimPoulter  wrote:  ``Its just coping the value from once cell to another``

The sudden appearance of the circ ref error has less to do with what is edited, and more to do with what is recalculated.

For example, if the Excel file references external data, differences in the external data might cause Excel to discover a circ ref due to a conditional formula.

Consider the following in a new workbook:

In A1, enter the formula =IF(ISODD(D1),B1,C1) .

In B1, enter the formula =A1 .

Because D1 is empty (it's a new workbook), ISODD is FALSE.  So Excel evaluates the false part that references C1 (no circ ref).

Now enter 1 (any odd number) into D1.

When Excel recalculates A1 (because it depends on D1), ISODD becomes TRUE.  So Excel evaluates the true part that references B1 (circ ref to A1), and it reports the (new) circ ref.

I say a "new" circ ref, because if we now replace 1 with 3, Excel does not report the circ ref again, even though A1 is recalculated, and B1 is still a circ ref to A1.

Presumably, the reason is:  Excel "remembers" that it reported the circ ref in B1, and that state has not just in the interim.