HP-UX ... PA-RISC

IRIX, MIPS, HP-UX, and PA-RISC all failed because of one guy.

When Rick Belluzzo became executive VP of their Computer division he was a strong advocate of killing HP's investments in HP-UX and PA-RISC in favor of Windows NT on Itanium before the latter was much beyond doodles on a napkin.

After killing HP's high-end computing, he went on to become Chairman and CEO of SGI, where he killed IRIX and MIPS in favor if WinNT on Itanium, getting the press to write articles about him like "Microsoft man's shadow over bankrupt SGI".

For such impressive^* accomplishments, he was then rewarded with a Prsident and COO of Microsoft job (in charge of their MSN division).

^{* And those accomplishments are indeed impressive. He successfully killed 2 of the 4 leading 64-bit computing architectures at the time [the others were AIX/PowerPC and SunOS/Sparc] in favor of a platform that hadn't even launched at the time}