We'll take some heat for this one, but we'd toss the Cell Broadband Engine on this pile as well. Cell is an excellent example of how a chip can be phenomenally good in theory, yet nearly impossible to leverage in practice. Sony may have used it as the general processor for the PS3, but Cell was far better at multimedia and vector processing than it ever was at general purpose workloads (its design dates to a time when Sony expected to handle both CPU and GPU workloads with the same processor architecture). It's quite difficult to multi-thread the CPU to take advantage of its SPEs (Synergistic Processing Elements) and it bears little resemblance to any other architecture.