Here's the problem:
=ISNA(S2) works fine (the result is TRUE)
But when I replace S2 with the formula (pasted in) from cell S2 and
press Ctrl+Shift-Enter, it breaks. Excel tells me "The formula you
typed contains an error." and won't let me proceed.

The formula is S2 is this (it is a single-cell array formula, of course
I don't type in the curly braces):
{MATCH(M2-N2,SMALL(Unrounded_2007-Rounded_2007,ROW(INDIRECT("1:"&ROUND(ABS(SUM(Unrou nded_2007)-SUM(ROUND(Unrounded_2007,0))),0)))),0)}
In case it matters the named ranges a
Unrounded_2007 = sheet!$M$2:$M$40
Rounded_2007 = sheet!$N$2:$N$40

Any advice much appreciated. Otherwise I am going to have to give up on
using arrays and write some VBA for this problem.