That is Scott 136 New Caldonia (1928), and Scott makes no mention of any such error. I have a New Caldonia collection as well and I have never seen or heard of that. I think Chesham85 and Mirman are very likely correct, and that "error" is just another stamp that got too close and too damp. Sorry. but if you've got the bucks and and want to be absolutely positive, you could expertise it, but I think it would be 99% likely to be a waste of time and money.