OK, I may have blogged this one before but....
Sometimes, when working on intonation with my students, I find that they are unconsciously lipping notes up or down.
Recently, when a student struggled to lock in the high G-flat, I played the chord on the piano and asked him to gliss around before stopping on the note. Essentially, I wanted him to play with a very wide and slow slide vibrato before settling on the note.
Here's my thinking: when we gliss around, we are less likely to lip notes. Instead (hopefully) we focus on getting a full-centered sound. Then, when we try to lock in the correct pitch/position, we are truly tuning with the slide as opposed lipping the notes up and down.
The psychological power of muscle memory is quite amazing. Even in my own playing, I find my ear arguing with my arm.
EAR: Dude, you're sharp. Bring it down.
ARM: No way, man. Second position is never that low!
And thus perhaps the lip begins to take matters into his own hands (don't think about that concept too much, it's just an analogy) and bend the notes.
Maybe they bend into "tune" but they also bend out of resonance.
Hey, we're playing a big tuning slide here...might as well use it.
Maybe if the arm had an ear of its own...