I have encountered a bizarre Excel 2003 bug that crops up from time to time.
Sometimes, a formula that is typed into a cell does absolutely nothing, i.e.,
Excel does not evaluate it. If I type the same formula into another cell and
then do a copy and paste, the formula works in the same location where it
previously failed to work. Here's the latest formula that I was using
where I encountered this problem.

=SUBTOTAL(103,$A$3:$A$64000)

I've verified that the cell format is "General" in both cases.