If the cell got formatted as Text (format|cells|Number tab|Text), then you'll
see the formula, too.
It would be like entering a formula with a leading apostrophe '=a1
Excel will treat it like text.
And to make matters slightly worse, if A1 were formatted as text and you put =a1
in a cell formatted as General, you'll see the that formula evaluates
correctly.
But if you edit that formula (F2, enter is enough), then excel will think that
you want that formula cell to inherit the numberformat of the cell it's refering
to.
This is useful for things like dates, but not so useful for other things.
And the only way I know to fix it is:
format the cell as general, f2, enter
(or format the cells as general and follow Peo's tip if you have lots of them.)