Formula not calculating

yes, "calculation" is set to automatic...

I occassionally run across a problem where a simple formula or lookup, is not updating. If I select the cell to edit and hit "enter", it will work. The formats are set to "accounting".

I know the quick fix: find/replace "=" wit "=". But need to know why this happens...b/c I'll always have the question of "what else isn't being calculated???"

I suspect that is has something to do with coping sheets with formulas, but I don't know. Does anyone know of this bug?