Revelation of the day

Filters not only change a waveform by attenuation, but distort it by individually phase-shifting the harmonics within it.

Duh, I'd done lots of freq/phase response plots in the feedbacks course at Tech, but the moment of enlightenment comes when you take that to the real world. When doing multitrack recordings where one mic is closer to a source than other mics taking that same source there is phase shifting.
Then in mixdown, to compensate we delay the signal from the closer mic until it sounds better (Another way to do so could be to measure all the distances and manually calculate the delay times, but that's not very practical when you have a band waiting to record). Then the equalization begins on the individual tracks using all sorts of filters. Last week when doing this in practice I'd totally forgotten about the phase after setting the time delays....but if I would have remembered the feedback course I would have realized that when adding the equalization, phase shifts are being introduced.

Bottomline: you need to have a good ear to be a recording engineer.