VFR flight ends at 17,999 ft. FL180 and above is PCA. When climbing or decending thur FL180, the altimeter is reset to what is required by the FARS. With a hair sitter on your jumpseat, before climbing thur FL180, you should be resetting your altimeter to 29.92 and have 29.92 in that box before the needles point to FL180. When assigned an altitude during IFR flight, during cruise at that altitude, your altitude may vary by up to 300 ft. before an ATC controller is required to report the violation. Controllers are friendly and will call you when they see a smaller difference. Remember for IFR flight in controlled airspace, instruments need to be checked within certain time frames.
At 500 ft. plus or minus the assigned cruise altitude, the report is done by the computer. When the computer does its report, it can't be turned around, until the authorities deem what the verdic shall be. In this case, a NASA report should of been filled out. Also, when you land, ground will call you, give you an number, and they ask you to call the ATC center. A logbook write up for a malfunctioning electric altimeter might be considered an appropreiate entry. Filling out a NASA report, on your part, is a good idea. No biggie, and no one will show up with leg irons and haul you away. Several hundred feet above FL180, most likely didn't even show up on their radar (high altitude sector).
Flutter limits for sailplanes are set by True Airspeed. I have an AS glider. At 16,500 ft. its 133 kts IAS VNE. At sea level its 154 kts IAS VNE. A placard by the A/S indicator is required for high altitude flight, which AS considers above 10,000 MSL. A design flutter speed test to 173 kts. has been done. If you happen to forget, and do 154 mph indicated at high altitude, the design flutter limit of 173 kts will not be compromised.
Last summer, out of Ely, I was cruising at 17,500 or so. Going SW bound, under a street, south of Eureka, its on the OLC, now that was a ride for all of mankind.