Too much phase inversion though and the mix changes dramatically in mono. Generally you only get phasing problems where you've done anti-phase based stereo expansion stuff or you've got a really weird synth patch.

Sometimes if you're using a lot of outboard you coudl have a cable arse about tit in which case you may get an almost total cancellation if you're running from a stereo source, these you can usually hear if you've got speakers set up half decently as it'll sound a bit sick and disembodied.

Wicked fun to use that kind of trick to make a sound sound like it's touching the back of your head but don't do it on an important soundin the mix 'cause like G says some (many) rigs are run in mono and it might just vanish.