Excel 2003 (and earlier versions)
I frequently run into the problem where I type a formula in one cell and it
doesn't work (all I see is the text of the formula in the cell), yet I can
type the exact same formula in a different cell on the same spreadsheet and
get it to calculate.

For example - I'm trying to use this formula (where XA:XZ represents any
random range):

=SUMIF(XA:XZ,"750000")

And it won't work in all the cells. I've tried clearing the contents and
also deleting the cell, but nothing works. Any suggestions?