#NA's resistant to defeat !

I need to put this in context.

Find file attached.

It started with a fairly straight-forward array problem. The OP solved it themselves, but I wanted to try something different “off-post”.

http://www.excelforum.com/excel-form...rom-range.html

Hopefully that will describe what I was basically trying to do…..along with some other “stuff”. My solution generated the desired outcomes successfully. It was the NA errors that had me stymied.

In column A the data.
In columns B:C the desired outcome.
In columns E:F my first error-trap attempt (failed).
In columns H:I my second failed attempt and in columns K:L my successful attempt.

My attempt to suppress these with IFERROR failed. An attempt to apply a LOOKUP / CHOOSE strategy I picked up in another recent thread didn’t work either…..but together they worked!

This example is probably not a good representative of my question. To be sure having a better designed formula in the first place is the better solution in this case, but can anyone tell me why some #NA errors are so resistant, and why this nested attempt worked when the others wouldn’t?

Any thoughts?