Liz and I flew to Factoryville, PA to have the plane modified to accept a detachable windshield deice plate. During preflight I moved the altimeter from 900 to 650 feet (Middletown's elevation), or so I thought. The altimeter actually read -100 feet and I changed it to -350 feet. High pressure had moved in and moved the altimeter from 650 to -100 feet. The analog display of the altimeter makes this an easy mistake to make.
It was a clear day with some tailwind so I decided to go up to 17,500 feet. Near Pittsburgh, PA, I listened to an automated terminal information service (ATIS) for a local altimeter setting. Then I realized my altimeter was set 1000 feet too high. I had been at 18,500 feet for an hour. You are not allowed above 18,000 feet without an IFR (instrument) clearance from ATC. I descended to 17,500. Since my transponder is altitude encoded, I knew every traffic control center within 200 miles could see me on their scopes. Also turbine and jet aircraft can detect the transponder signal (with their TCAS systems). Two weeks later, I got the inevitable call from the FAA. They explained they had to send a reprimand letter because I had tripped several TCAS warnings that were written up by numerous aircraft. On the grounds that the violation was unintentional and no safety hazards were created, all I got was the letter. Another humbling experience.