Revision A

The Rev A. G5 tower had busted fan control firmware that caused the fans to spool up to maximum speed when running Classic and in some other situations, the Firewire 800 ports were broken, and there was the infamous chirping noise.
Rev. A iMac G5 had similar problems with fan control, and it had heat dissipation problems. It also had the capacitor problem, which wasn't Apple's fault.
Rev. A MDD Powermac G4s had faulty power supplies that made excessive noise and, I think, tended to fail often.
Rev. A Pismo had logic board problems, most often resulting in the sound chip failing.
Rev. A B&W G3 had a faulty IDE/ATA chip.
Rev. A PowerPC Performa (5200/6200) had faulty monitor connection, faulty logic boards, crashy sound control buttons and incredibly bad performance