Every single parameter on the data sheet is tested. The chip is
clamped in an expensive temperature control rig, driven with a clock oscillator,
and every pin timing is tested to make sure it meet spec in all corners of
temperature/voltage/clock schmoo.
The short answer to: "What happens when I overlock?" is: YMMV.
You are off the data sheet. All guarantees are off. In addition to more heat,
the first thing that will go is pin timings. Your design may have enough margin
where that won't matter. Or maybe it won't matter on a cool day, but a hot
summer day it gets flakey. Or maybe your design works today, but won't after
three years of aging. (Modern CMOS *does* slow down as it ages due to the
"hot electron" effect.) It may draw more supply current than spec. Or, maybe
only certain date codes work, but parts from marginal boats don't. Or maybe
it works with certain date codes of some other manufacturer's RAM chip if
they were made on a sweet day, but not marginal ones.
It may appear to work until you exercise one particular logic path that does
not make timing at the high clock speed but doesn't matter for common
operations. An example being the horde of idiots that overclock Intel Pentium
processors and say: "It boots Windows, therefore it works!" Clue #1: There
is no floating point in the Windows boot code. Clue #2: Windows does not use
the newer instructions in the OS, because it is compiled to run on old
processors.
You do not have the ability to test the part the way the manufacturer does.
If you get your kicks by overclocking, go for it. But please don't sell it to a
customer that expects a reliable system. And don't whine if it breaks. And
don't brag to Old Farts(tm) about it, it won't impress us. Having said all that,
if it just needs to hang together long enough to turn in a grade.... find a big
honkin' heat sink -- power dissipation goes up with the cube of clock speed.
