IsError() tests whether a cell value contains an error, not whether a function returns an error. That might happen when CurrentMember returns an error, but it's not quite the same...

The problem is almost certainly the part of the calculation that uses IsError(); when you say

[Measures].[Net Lost Commitments (during period)] =

iif(iserror([Date Dimension].[Fiscal Year Hierarchy].CurrentMember)

What you're actually...

But what makes you think the explosion isn't already happening? Actually, I think I can answer that question by picking up on another thing you said, which was that "I think...