I've made a lot of mistakes, and I hope I've learned from all of them.
My worst one: Landing without clearance. Was cleared into the control zone and told I was number one. There was zero traffic and I was unused to being told that so early, somehow confused me. Landed. When the wheels touched, I realized what I'd done and swore. Taxied off the RW, had to cross another RW so had to call tower, the female occupant asked in her sweetest voice: "Could it be possible that you accidentally landed without clearance ?". I said "ahem, that's entirely possible". And that was it. I guess she didn't report it, as she had failed to notice me land !