from a mic position perspective. if you simply stand just off center in a room and speak, then shift sideways to a wall, you'll hear a distinct shift in what you hear. this is because the sound is bouncing off the wall and into your ear and mixing different frequencies at various phases as you do it. probably the simplest example of comb filtering. no equipment other than your ears needed. however, your ears will try to compensate for it, so the effect is short lived. a microphone has no such compensation and it will faithfully record the effect - not good unless you want it.
from a speaker perspective, the bouncing not only introduces cancellations and reinforcements, but changes the loading on the speaker which can cause the power needed to drive it to change and result in additional distortion. depending on where the speaker in placed front-back, side-side, has different effects on its response which is one reason for having adjustments on the speaker to cut or boost bass. a baffle mounted speaker designed for open mounting will experience a bass boost, and depending on how the baffle is constructed, you could also shift the apparent crossover points in a bad way.
Seigfried Linkwitz and Phillip Newell has some excellent treatises on this subject...