Sure, but if consequences are the measure of whether you've made a mistake it's hard to enact any kind of preventive strategy. the question is posed in the present perfect, after all.
How do you know when you've made a mistake?

Sure, but if consequences are the measure of whether you've made a mistake it's hard to enact any kind of preventive strategy. the question is posed in the present perfect, after all.
You are basically asking a question like, "am I guilty of doing anything wrong, if I don't suffer the usual consequences for such an infraction". Whether we make it moral or chessically semantical, you can hope you don't get caught but, I will spare you what you already have read about playing hope chess...
... you can argue over whether trying to use your feelings has any merit, if that is where you were trying to steer this, to make me seem contradictory but, I wasn't advocating playing bad chess, if you get away with it.
I was advocating training yourself to be aware of your feelings and responding to them appropriately, as opposed to completely ignoring them or letting them drive you nuts. I personally think the energy and focus one can get from their feelings, can be more powerful than one's brain alone can be...

A tip I once heard for finding mistakes when you're analyzing your game:
Every five moves, evaluate the position (+/=, =, -/+, etc.) If the evaluation changes, someone made a mistake within the last five moves.
I've done this and have found it very helpful.. There's a lot to positional evaluation, but 5 easy things to check are:
- The safety of the Kings
- The material
- The activity of the Pieces
- The pawn Structure
- Who is better after a queen exchange.
*in that order.
In addition to this.. if you write your OTB games down and do a post-mortem review with your opponent you'll learn a lot about the mistakes you've made.. you could even do another review with a stronger player (after you analyze on your own)
This is something you develop a feel for over many games of experience, normally people do not understand their mistakes untill after looking at a loss and thinking about which move made it hard to stay afloat.

If you make a mistake and your opponent doesn't capitalize, was it still a mistake?
It's a mistake that failed to reach its potential.

Sure, but if consequences are the measure of whether you've made a mistake it's hard to enact any kind of preventive strategy. the question is posed in the present perfect, after all.
Yeah, the question doesn't seem to be very useful. I think he was mostly wondering how very strong players judge their moves to be a mistake when they're clearly not dropping pieces.
If you make a mistake and your opponent doesn't capitalize, was it still a mistake?
Yes, not everyone one will fail to take advantage. Why not avoid that possibility, if you can ?