I have done some very bad things.
I must say, rarely with classical music, but more often with popular music, such as themes from movies, etc. Cases in point . . .
Once and a while I hit a chord that I just don't like. Sometimes it sounds like a clinker to my ear. so I just take a pencil and write a chord that sounds better. Sometimes my piano teacher even approves of the change -- with the original chord she may frown and say, "you played it right, but it sounds wrong."
Since I just play for my own enjoyment, I will sometimes take a few liberties with classical pieces as well. for instance, if a series of notes are marked stacatto, I may just not play them that way.
And I often play difficult material more slowly than marked, simply because that's the only way I can play it, although it does speed up somewhat with practice.
"Let he who is without sin cast the first stone." [Or she . . .]