There are lots of possible ambiguities in major/minor relationships which can make for some interesting harmonies, but which can also - as my original post suggested - make for some interesting dischords.

An example. I play a solo fingerstyle version of The Cuckoo's Nest in G (fits nicely under the fingers on guitar). The common opening chords to this are:

G / / / G / / / G / / / G / / /
Am / / / Am / / / Am / / / Am / / /

I actually play:

G / / / G / / / G / / / G / / /
Am / / / Am / / / F / / / F / / /

which is rather a nice shading in the 2nd 4 bars and a bit more subtle than a solid Am all the way through. The subtlety, of course, is the in the substitution of an F note for an E note in the Am and F major triads.

It's fine as a solo but I might hesitate at playing that particular change if others were involved...