Scott, if a stereo recording is made which has poor mono
compatibility, and that recording is played in mono, what
problems is one likely to hear? I mean, what will the
problems *sound like*? Drop-outs? Garbled? Harsh?

Well, if there are phase and arrival-time differences between the channels,
what sorts of audible effects would occur when the channels are summed?
(Fill in the blanks.)

Even intensity-only recordings have mono-compatibility problems. A centered
signal will rise in level 6dB, while those to the extreme left or right will
show no rise. The result is imbalance -- the closer a sound is to the
center, the louder it is in mono than it is in stereo playback. Some
engineers introduce broadband phase shift for centered and near-centered
signals to reduce this effect.