You're not hallucinating, and what you're seeing is a result of the compression used in HD.

The compression method is "MPEG", which consists of JPEG applied to individual frames, plus differencing between frames, so that only areas that have changed from one frame to the next are transmitted.

Both your mine shaft and talking head examples sound like the classic case of part of the scene remaining relatively static while another part must constantly be updated. Comparing it to "poorly superimposed" bluescreening is exactly right. If the mine shaft opening or the talking head moved slightly relative to the background, there'd be a "halo" area at the object boundary that constantly needed updating frame-to-frame, producing what might have looked like "writhing" at the edges; while the object center, and possible the background, remained static.

MPEG, like JPEG, relies on fooling the human visual system into compensating for these artifacts. But I've noticed that everybody's overcompressing video to try to cram more channels into their allocated bandwidth, and pushing it to the point that you can see the degradation. When ComCast switched from analog to digital, most pictures got worse because they overcompressed to wring the maximum revenue from the fixed bandwidth of their antiquated coaxial cable. I still get picture breakup and pixelization. HD can theoretically provide great picture quality, but only if the broadcasters and purveyors are willing to let HD be HD. Profits will tend to trump quality every time.