These are definitely some of the more frustrating design job moments for me, when this occurs. Particularly because, usually unwittingly, these butter-inners have in effect undercut you as the professional in regard to the task at hand, leaving you at least momentarily to stand there awkwardly, nodding/smiling of course, until you see a chance to steer the design input back to the original framer-client state.
Personally I recall one particularly complicated piece I had spent a moderate amount of time, building rapport with the client, establishing the purpose and goal of the framing/where it was going to be, etc. After a few almost moments (not quite at the awestruck framing moment, but skirting the edges), some interior decorator elsewhere on the counter felt it was her duty to share her advice on how to proceed with the design, much to the chagrine of myself, who stood there, shifting my weight until she was done with her say. Fortunately, my coworker had wrapped up the decorator's order/purchase, and she left soon after, upon which time my customer looked at me, and stated in a low voice "I really don't like what she did at all." I smirked, and continued on after a shared chuckle. Got a nice sized frame, double mat, fillet, and museum glass piece, despite the interruption. Unfortunately, that was by far the exception in this situation.