Undergraduate shenanigans in the 1980s: The IBM PC was underclocked at 4.77MHz (as I recall) but rated at 8.00MHz, thus the "turbo" switch on many clones. Our little company had secret early-access IBM AT to develop software on as part of IBM's Scientific Software Suite. IBM paid to have a security system installed in our rented suite, and required that the machine be in my windowless office in a larger windowless back room. Was the AT also underclocked? The system frequency was controlled by a two-legged crystal soldered in, so I purchased a number of such from an electronics surplus outfit, soldered and unsoldered until I reached the maximum stable speed. It was quite a boost (I seem to recall that it was the video subsystem that was the ultimate bottleneck). When the AT eventually went public I made a few bucks hot-rodding them. Oh, and when IBM wanted their loaner back I did put the original crystal back in--only one coworker knew about this, and certainly not my bosses.