We had one on a GDL-82 installation that was interesting. Baro failure.
The altitude was coming from an encoding altimeter. It had ONE incorrect
altitude. 5500 feet reported as 6100 feet. So climbing thru 5500, it outputted
5200, 5300, 5400, 6100, 5600, 5700, etc. The ads-b reporting checks that each second the altitude change during that second makes sense. A 100 or even 200 foot change would make sense. But 5400-> 6100? Couldn't happen. So it gets flagged.
5500 is not an altitude that often gets checked. So a single altitude error like this may pass the IFR check but get caught by the ADS-B checking.
I checked several encoding altimeters. -- all United models-- and two others had similar errors, one at 2500 ft and one at 3500 and 3600 feet. Must be a cootie on the altimeter optical encoder wheels.
we installed a TCI nano encoder to resolve te problem.
Bill Hale