I have 3 Xmega128a4u chips and one 256a3bu. One of the 128a4u chips is different from all my other Xmegas.
All the others have the 32kHz RC oscillators calibrated correctly at the factory. Apparently the "correct" setting is within 0.25% based on what I've seen and in the observed granularity of the frequency adjustment. This strange chip ran 1% slow. That wasn't a big deal. At that accuracy the USART and USB ran fine. But being a perfectionist (occasionally ), I bumped up the frequency adjustment by 2 to give it the "correct" frequency. I stored this setting in the User Sig Row and I apply it every time after reset.
I subsequently used that chip for software development for several months and all was well. Then about 10 days ago something happened. I found neither the USART or USB was working. Then I discovered the 32kHz RC oscillator was running 5% fast. This change happened that day. I use the USART and USB every day and I measure the RC oscillator frequencies often. This change didn't sneak up on me gradually.
I set the frequency correctly again by decrementing the freq. adj. setting by 16. I stored this new value in the User Sig Row and it has been working fine ever since.