In AVR datasheets under the Electrical Characteristics section you will typically find a graph like this (this one is from the ATMega328):
I've seen designs that seem to "work" but operate outside the shaded envelope. Specifically, I've seen 3.3V (Arduino) designs that run the clock from an external 16MHz crystal. Clearly, this is out of spec. What are the practical negative consequences of running outside this envelope?