Funny this topic has come up again. We've been gettin a few worm tracks here and there.
But to Milton, yes, my welder's last weld busted RT (ASME sec VIII spot RT) due to these tracks. They were not visible on the surface, but the RT indications were a dead giveaway of what it was. Anyhow, they were rejected as slag inclusions, but that's not what they are. ASME criteria for spot RT mentions something about slag "or other cavities"...so yes, it was rejectable.
Also, it could have very been rejected for an elongated indication, had that terminology been mentioned by the criteria. But again, it was not visible on the surface. Seems to have not fully come to the surface!