A fully committed play-and-miss might be a bigger mistake than a nick, but not one where you go on to make a stroke, realise at the last fraction of a moment that you might nick it and then decide to lessen your commitment slightly to make sure you miss it, just. (I'm not talking aout withdrawing from the shot completely at the last moment, I'm talking about those cases where you don't even have the time to withdraw from the shot - you just slow down your hand movement slightly, slow down your bat speed slightly at the last moment to miss it - if you understand what I mean).
And from personal experience, I would say it's very difficult to distinguish between the two from a spectators PoV. I would say more than half the play-and-miss that we see are of the 2nd type.
"I want to raise my hand and say one thing. Those who complain about my love for the game or commitment to the game are clueless. These are the only 2 areas where I give myself 100 out of 100."
- Sachin Tendulkar, as told in an interview published in Bengali newspaper Anandabazar Patrika after his 100th International century (translated by weldone)