Witness Paint / Torque Seal
I have not seen a properly tightened B-nut back off, ever... But I have seen an improperly tightened B-nut come loose with catastrophic results. The Torque Seal was applied in such a fashion that it stuck to B-Nut and overlapped the fixed portion to give the impression that nothing was wrong. It is a thick product and sometimes gives erroneous results. A second look (Independant Check) was completed on the affected line which had Torque Seal applied on a finger tight B-Nut and all was apparently done by the book. A few hours later the line backed off with the Torque Seal still visible on the B-Nut.
Not the most reliable system.
Paint in this case would have been better but it is not always easy to apply in all situations.
Nothing is foolproof.