▼Concepts: Phase Cancellation
When you have a wave that's 180 degrees out of phase with another, they cancel each other out. This can happen within the mixer if you have two microphones panned to the same output channel. More expensive mixers have a phase shifting knobs to avoid the problem.
Sound waves traveling through the air can phase cancel too. A speaker's sound wave bouncing off a wall can return and hit the wave coming from the speaker, and if they are 180 degrees out of phase the sound is gone completely. To spot it while mixing you use pink noise through the system as it's the whole spectrum. When you're mixing, if you have to keep boosting a particular frequency range, it might be the problem. You can see it on a graphical meter also.
Phase Cancellation in your Room
Your control room will probably cancel some low frequencies. To find out which you can do something called 'freq'ing out the control room' which is done to find the dead bands. If there are dead bands of frequencies it'll show through into your recordings. To fix the problem you use baffels or sound proofing material (it's why most studios are so heavily padded).