Microphone bleed

Sit two people at a table with a microphone each and both microphones hear both people. The unwanted voice arrives slightly later and slightly duller, so cutting one channel leaves a ghost of it on the other, and any processing applied to one voice acts on a smeared copy of the second.

Distance and polar pattern do most of the fixing. Engineers use a 3-to-1 rule: keep each microphone at least three times further from the other speaker than it sits from its own. Editors handle what remains by gating each channel or ducking the inactive one, which works right up until both people talk at once.