One of the owners of Gryphon Guitars, Palo Alto CA contributed an article to Acoustic Guitar magazine a couple of years ago that listed the 10 songs NOT to play while trying a guitar (because everyone else has played them so often and so poorly that the store owners go bonkers when they hear them). The list included: Blackbird, Stairway to Heaven, and I've forgotten the other eight.
I realized a long time ago that I could never own a musical instument shop because I would wince every time someone picked up an instument; would have an anxiety attack every time someone lightly bonked an instrument; would come completely unglued every time someone bonked one hard.
When Ginny bought me a Goodall guitar the other day, even thought it was a midweek morning, each time I was doing a serious "final test" between two or three choices of Martins, Santa Cruzes and my Goodall, someone would suddenly be seated on the chair next to me, playing good or bad music on another Martin (or worse yet, on one that was still in the running to be mine). Drove me crazy. I almost scrapped the whole idea. Ended up doing several pilgramages to the owner's office to sit in a quiet space. Damn, I'm becoming a difficult old codger.

Bark Woof