[spion]

I don't understand why its usual to assume that the only two choices are

  * show N/A everywhere
  * silently ignore sources of potential errors.

Why not make the calculation while adding a visual cue (e.g. a yellow or red indicator in the corner) that marks the field as tainted? Now the users get to decide whether they want to investigate or ignore that.

[ben_straub]

You mean like Excel actually does? http://i.imgur.com/kMPKnqO.png

[dllthomas]

Does that propagate, or is it only one hop?

[spion]

Yes.