A rather naughty topic - a bit mischievous; but I admit sometimes I have hit the wrong button, or key, and unintentionally. After all concertinas have small studs for buttons, and are, generally, close together, to manage all the time; at speed often!
But then there's that moment of mischief occurs in the mind of performer that says .. "Go on be daring, have some fun, and ditch the rigid rules"... and the discord can then resound! Much to the aching ears of anyone nearby!
You will all have that same experience I am sure of that clashing note moment [which you can say is 'quite deliberate'] .. but they are interesting when they occur, and concertinas, particularly, to me. due to their timbre of sound, lend themselves to this effect very well [intended or not].
It can also be quite effective for atmosphere, or to emphasise a weird theme maybe?
Although I try and make notes sound nice, I have now and again got going with the discord too, just for the devilment of it?
Any one else had similar 'Rebel' moments between the sweet music?