Well, one big myth that "engineers" like to throw around is that "digital is digital" and "ones and zeros are ones and zeros."
This just isn't the truth. Any engineer worth his weight in calipers and diodes, and I don't claim to be a great one, knows that ones and zeroes are determined by voltage levels and thresholds. Interference can easily flip a "one" to a "zero" and vice versa in the real world. Even on paper it happens, and it certainly happens in lab testing environments.
Does it affect our systems? Honestly I'd have to measure it myself to say for certain. Will you ever notice a single bit in an audio and/or video stream being biased improperly? Absolutely positively not.
When "engineers" (computer engineer here) say that 1s are 1s and 0s are 0s, they mean it. Of course it's still an electrical signal coming through the cable that is susceptible to EMI, but the fact is that digital transmission over an electrical cable is the only way to transmit a perfect signal from one end to the other. Even with relatively low quality cables, even if there is noticeable and measurable signal degradation, it's relatively easy to transmit a perfect audio or video stream.
I mean, you have a 30 gauge HDMI cable at 50ft, there will probably be enough signal loss to prevent accurate transmission of the signal. It's not a matter of the bits "flipping." What'll happen is that the square wave (which has imperfections of its own like propagation times, etc) will stop looking like a square wave and start looking like random spikes. The great thing about digital data though is that it can allow for flow control, error checking and other sorts of signal integrity checking without disrupting the perfect signal. So basically, if you get a picture or sound through HDMI or optical, you're assuredly getting the HIGHEST possible quality you can get. THAT's the point, you will not get 95% or 90%, it's ALL or NOTHING (pretty much). THAT'S why some people say 1s are 1s and 0s are 0s. (it is possible to lose frames or see visible imperfections in video but that is something that will be very obvious and is an attempt to prevent a complete loss of signal)
Paying any more than $10 for a 10ft HDMI cable is overkill. Fact is a decently made digital cable (i.e. cheap) is MORE than suitable for even the most expensive home audio/video systems in terms of fidelity.