As a tracking engineer, one area with great potential for out-of-phase material is multi-miking single instruments. To mitigate this, I am sure everyone is familiar with the 3-to-1 rule:
Two microphones intended to pick up one source must be placed apart at least three times the distance that either micro-phone is from its intended sound source.
This rule works well but there are many ways to control phase and comb filtering effects. Say, for instance, you are picking up a solo acoustic guitar. Using a coincident or near-coincident pickup will most often be better at minimizing phase problems. When miking a snare drum (or any instrument) from both sides, you can control phase and comb filtering using two different micro-phones at varying distances from the drum. This can also be very effective for tone control. It is worth noting that miking any source from front and back simultaneously may invert the polarity of the signal in the rear mic. Both front and rear signals will best combine, especially in bass response, if you compensate by phase-inverting one of the signals while recording.
A useful technique to check for phase problems with the aforementioned methods is to invert the polarity of one of the mics and then adjust its position while listening for the smallest combined level. When you switch the polarity back to normal, the combined signals should sound with the least amount of phase cancellation.
This technique will also work when using spaced pair configurations on large stereo pickups. Compared to near coincident pickups, spaced pairs, while sounding more spacious (varying degrees of comb filtering), do not always degrade well to mono. When setting up your spaced pair, monitor-ing in mono and adjusting the distance between the microphones will affect the tonal balance of the mono pickup. Subtle adjustments will make substantial differences to the mono mix without affecting the stereo sound significantly. Finding and maintaining an even sound while moving the microphones is easy as you switch between mono and stereo monitoring. This is often referred to as “matching microphones for phase.”
As is the case with most things audio, a solid understanding of the science and a respectful adherence to the rules is always tempered with a thought-ful listen to the result. Pay close attention to all the subtleties in tonality and you will train your ears (and eyes) to fully appreciate all the nuances involved in achieving great stereo through better mono.