Evaluate Sub-formula Only Once

Prior to XL2007 you can ape IFERROR via a UDF.

Alternatively if as implied in OP the output of the formula is a consistent data type (eg number) then you can also adopt an approach along the lines of:

=LOOKUP(9.99E+307,CHOOSE({1,2},0,f()))

here the f() formula [whatever it is - we assume non-UDF] is evaluated only once.

for a text output with say default of Null you would use:

=LOOKUP(REPT("Z",255),CHOOSE({1,2},"",f()))
(assumes f() will not a return a string > 255 chars)