unknown #value error
 

I have a formula in a column of cells in the form (this is for row 18)
=-IF($D18="X",0,RMClash(B18,X$9,Y$9)+PLClash(B18,X$9 ,Y$9))

where RMClash() and PLClash() are user functions. The functions work just
fine, and when I do Evaluate Formula in the Auditing menu, it doesn't show
the error until the very end. The last couple steps a
-If(FALSE,#N/A,0)
-0
#VALUE

However, all of the formulas are identical, and only some of these have this
problem. If I edit the formula (without making any changes), it gives a
proper value when I press enter, but returns to #VALUE when I do a
recalculate.

Has anyone seen this before? How to prevent it from reoccuring?