Any thoughts on why it happens? I mean I know it happens because smart
cut and paste is enabled, but why do you suppose that is?

Are you able to identify the line that throws the error?

To hazard a guess as to cause: from your account, all cells are actually
processed, so the error comes at the end of the loop. Perhaps the smart
cut-and-paste, in trying to deal with the superfluous space, is shifting an
internal range pointer, so that on the final iteration it thinks there is
another cell to process, then throws the error when it finds there isn't
one.