Pentium IV

With the Pentium IV, Intel rearranged some of the meanings of return values of CPUID, most notably abandoning the sequential "family" number. Pentium IVs (and higher) return $F or 15 in the family field; Intel says this signifies that "extended family information is available", in reality what it does is cause poorly-written software to crash. Most notably, Linux servers & operating systems installed with Pentium IVs originally failed to boot, not understanding the "1586" CPU they were presented with (expecting a single-digit family like 686 or 786).