jboyton:
The stock Uno R3 has both a crystal and a resonator. The Atmega8U2 uses a crystal; the Atmega328 uses a resonator. In addition to being less accurate, the frequency of a resonator is also more temperature sensitive than with a crystal.
Yes, that's the worst part of a resonator. They drift all over the road like a drunk driver as the temperature changes.
Plus, I wish the Arduino used a clock source that was an integer multiple of standard serial baud rates. The 16.000 clock results in an actual baud rate of 9615 for 9600 (+0.16% error) and 117647 for 115200 (+2.12% error).