Since security is not the issue, I believe it would be a mechanical/maintenance or another operations error. My brother was a metallurgical engineer at a few plants, including my first...VY.
He was emphatic about stainless not being contaminated with tooling that had been used on carbon containing materials. I have been to many BWRs recently that have exhibited rust on stainless, mainly near or on welds and joints. The carbon contamination may cause micro fractures and when combined with internal corrosion, a greater possibility of failure. There was a period in the 80's and 90's where there was a required amount of piping to be checked for corrosion / wall thickness. I haven't seen as much in recent years though. Through complacency, lack of vigilance, or "cost effectiveness" these programs have been dismissed. It also seems for quite some time, if things are screwed up or "don't seem right", nobody really wants to hear about concerns, really. Sure there are "concerns" programs, ran by the companies you're working at. Do they really take concerns to heart? Sometimes perhaps. I will bet the concerns I have pointed out, through the years, ever got fixed. A few of these were on safety systems. The plants really just don't care.... "Button 'er up and pull the rods", until the next time. But I digress....