The Pentium FDIV bug was a bug in the Intel P5 Pentium floating-point unit (FPU). Certain floating point division operations performed with these processors would produce incorrect results. According to Intel, there were a few missing entries in the lookup table used by the divide operation algorithm.
This flaw was found in 1994 and verified by other people around the Internet, and became known as the Pentium FDIV bug (FDIV is the x86 assembly language instruction for floating-point division). One example was found where the division result returned by the Pentium was off by about 61 parts per million.
The calculations the Prime95 makes are effected by this bug and some users ran Prime95 to test their machines for it.