Phasing-induced shifting issues can run wild. Or, if carefully planned, can be taken advantage of.
One particular instance for me was a persian music ensemble, of a perisan fiddle, setar (long-neck lute), and two hand drummers, one on either side.
Now, to someone not at the concert, and not familiar with the music, what happened would not be noticeable. What did happen was that the drummer to the ensemble left (stage left). His entire kit stayed in the right channel, except for one particular drum, which was in the monitors louder than the others of his kit. The sound guy placed a monitor with my mics in front of the monitor, after I had already set up, and gone to dinner. When I returned, it was too late to do anything about it. Its presence dragged that single drum from hard right, to being oriented to about 85%+ in the left channel, completely across stage from its actual presence.
No one else but myself, and the artists would know it, but, it drives me friggin nuts.
The drummer closest to the camera,... his drum came into the wrong channel, and sounds like it came from across this stage arrangement. You can sort of see my stereo pair in tight to the center players, about 3' out, centered between them..