...Chopin spins in his grave...

Playing Beethoven from the wrong side of the organ, if you catch my drift, standing up on the other side from the keyboard, leaning over. Clever, yes. Useful, um,less so.
And, to sneak in an obligatory RT reference, but not unique to him, the fiddling with the tuning to alter a note, by turning the tension on the strings up and down at the neck.
The drummer in Alberto Y Los Trios Paranoias played a solo whilst slowly deconstructing his kit (kicked it all away, basically)
Dick Heckstall-Smiths 2 saxes together, bettered only by Roland Rahsaan Kirk, who could mange, apparently,3.
I hate pianists whoplay the strings like a harp. Godawful tedious "free" jazz impro concert with Keith Tippett comes to mind, really should be in worst concert ever strand.......

...but I once saw John Lewis of the Rimshots change a string in mid-song whilst he was still actually playing it.

I've seen video footage often of Stevie Ray Vaughan changing guitars mid song when a string broke, but I'm sure there is also footage of his roadie/guitar tech coming onstage and changing a string mid song.

a memorable stunt at 2mins...

using a pint of lager as the slide......small local support band a few years ago. Can't remember who, where or when but I can remember being impressed.

Also, there's a guy called John Smith (yeah, honest) who has a song called "Winter" where he uses his acoustic guitar as a percussion instrument. I remember hearing it for the first time and wondering how he did it.

This clip looks pretty good, although it is black and white. He's toured a bit supporting John Martyn over the last year or two.

once supported Pussy Galore and they had a fully miked-up Harley Davidson as a drum kit. We felt like boys watching men.