It was supposed to be the game-changer. A Waikato businessman had solved the Bain murders by noticing something the police, scientists and defence experts had missed for nearly 20 years.

Two small marks on Robin Bain's thumb must have been caused by him loading bullets into the sooty rifle clips used in the killing of the Bain family on June 20, 1994, the claim went.

Robin Bain must therefore have killed himself after shooting his wife and children. David Bain's innocence was beyond doubt.

It was an interesting theory, worthy of examination, but perhaps opinions about its importance should have been withheld.

Instead TV3's 3rd Degree programme trumpeted the finding as though it was the clincher, the Holy Grail of the Bain case. The public could now be assured David Bain deserved compensation for his horrific ordeal.

Maybe 3rd Degree and the Bain camp should have kept their powder dry because, as was announced yesterday, further expert analysis raised doubt that gunpowder residue caused the marks on Robin Bain's thumb. According to police, fingerprint experts concluded it was strongly indicative the marks in the photos were "minor superficial damage to the skin surface". An ESR scientist concluded there was considerable doubt the shape, dimensions and colour of the marks on Robin Bain's thumb were consistent with marks made as a result of loading a cartridge into a magazine.

Predictably, diehard Bain supporter Joe Karam immediately called the analysis flawed and biased. Just like all the other analysis unfavourable to David Bain.

In my view, the claim the marks were gunpowder residue and supportive of David Bain's innocence was typical of many of the Bain camp's main arguments. A small fact, blown out of proportion by supporters whose zealotry, in my opinion, overwhelms good sense.

There is no shortage of examples. Take for instance the importance Bain's defence initially attached to the fact Margaret Bain withdrew $200 and transferred money to her credit card account late on the night before she and her children were killed. The Bain camp was adamant it showed Margaret Bain had been upset by reports of incest and had gone out to get some cash in readiness for upheaval in the family.

It turned out it was much more likely - in my view - that the very frugal Margaret simply wanted to transfer the money between accounts to avoid incurring any bank penalty after midnight on June 19.

No-one should be surprised at the latest development.

In my view, even a cursory inspection of the marks showed they were quite different in colour, measurement and look to the marks produced by the other simulations.

The Bain camp also failed to explain why only one pair of lines was visible when Robin Bain must have loaded many bullets into the magazine. Neither did they explain how the lines of residue survived Robin Bain handling the rifle before he supposedly shot himself.

So just a coincidence, then? 3rd Degree asked.

Well no. I think the recent expert reports suggest that the marks highlighted by the programme were most likely pre-existing damage or injury to the skin on Robin Bain's thumb as shown by the original fingerprints.

He had been trying to fix his spouting during the weekend before the murders.

The marks on Robin Bain's thumb should never have been the issue they became. They needed to be viewed in the context of all the other facts (and conclusions from those facts) in the case.

If that was done the Bain camp and TV3 might have avoided the embarrassment I think they should now be feeling.