Important:The bug that caused the N-1 primality test of 1024*3^1877301+1 to fail in PFGW and LLR was a serious bug. It is triggered by a lengthy carry propagation. I cannot prove that Mersenne LL tests are immune to this bug, although I've rerun dozens of Mersenne prime LL tests and every one passed. The bug affects AVX and FMA computers (more details in a later post). The bug has been present since version 27.1. To be safe, I recommend all users with Sandy Bridge or later CPUs upgrade to this version.