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?

How to make life more interesting 101:

If you don't care

that your results may sometimes be wrong,
that your system may sometimes crash,
that your life may be more interesting,
that your Segway clone only occasionally does face-plants for no obvious reason,
that ...
Then by all means run the part outside manufacturer's spec.
You get what you don't pay for.
If you have a $10 head, buy a $10 helmet.

It may often work.
It may not work sometimes.
It may not be obvious that it isn't working sometimes.
A divide may usually work
A jump may usually arrive.
A table may be looked up correctly.
An ADC value may be correct.

Or not

