it is usually very audible by loss of tone / bass and can also cancel out sounds completely from the mix. It can be measured with phase scope plugins on audio tracks in a DAW or with an expensive oscilloscope if analogue.

it is essentially when 2 tracks are at opposite polarity with the same sound characteristic (ie. timbre & frequency).

sry no time to test this out right now, maybe i’ll get round to it at the weekend. however, i’d be very surprised if this was the case and also not sure why you would want to combine individual outs with main outputs in the mix? Would also be useful to understand how you tested this / steps to reproduce…